++++++++++++++++++++ Date: Tue, 24 Jul 2001 23:18:02 -0500 From: "George, W5YR" Subject: Re: [Elecraft] Feedline for my 40 meter Inverted V Another point to keep in mind when dealing with high SWR situations is that voltage and current on the line increase above normal "flat line" values as the square root of the SWR. The effect of this can be to place relatively large voltages or currents at the tuner input and perhaps overstress some of the components. A lot depends, of course, on the feedline length and whether or not voltage or current maxima are present at the antenna tuner input/output. For example, in the 50:1 case cited below, the peak voltage on the line will reach over 7 times the normal value at some points along the line. Same for current in the line. 72/73, George W5YR - the Yellow Rose of Texas QRP-L 1373 NETXQRP 6 Fairview, TX 30 mi NE of Dallas in Collin county EM13qe Amateur Radio W5YR, in the 55th year and it just keeps getting better! Icom IC-756PRO #02121 Kachina #91900556 IC-765 #02437 Ron D' Eau Claire wrote: > > Phil asked: > > I have a 40 meter inverted v for my K2 (1264). The K2 also has > > the ATU. I am > > using an RG 58 as a feedline. ... > > Is it better to feed this antenna with say 300ohm transmission > > line and use > > a 4:1 balun at the end of the feed line. And if so why? > > I await an answer with baited breath. > > The answer depends upon how long the feedline is and which bands you are > operating on. Fifteen meters will be fine with the coax, since a half wave > on 40 is nearly 3/2 waves on 15. But other bands may show very high SWR's > on the feedline and the feeder losses may become very high. (The fact that > the KAT2 can match the impedance at the rig is a great testimonial for the > KAT2, but it does nothing to reduce the feedline losses. What determines the > losses in the feeder is the SWR at the ANTENNA. Whatever happens at the rig > end of the feeder has not effect on that.) > > For example, I did a quick check, plugging into "Dipole", a simple doublet > simulator, a typical 1/2 wave doublet on 40 and shifting the frequency to > 10.1 MHz (30 meters). On 30 meters the SWR on the feedline is something > above 50:1. ANY coaxial feed line will have very significant losses at that > high of an SWR unless it is very, very short. How big the losses might be > you can get by getting the loss figures for the type of feeder you are > using, the length of your feeder, and using the charts in almost any antenna > book. > > As far as 'balanced' feedlines go, TV 300-ohm ribbon is just about the most > lossy line you can get. Of course, it is also the most 'portable'. > > Ron AC7AC > K2 # 1289 ++++++++++++++++++++ Date: Wed, 25 Jul 2001 05:45:58 -0400 From: Charles Greene Subject: Re: [Elecraft] Feedline for my 40 meter Inverted V At 08:36 PM 7/24/2001 -0700, Phil & Chris wrote: >I have a 40 meter inverted v for my K2 (1264). The K2 also has the ATU. I am >using an RG 58 as a feedline. Works great...1:1:1 at 7.000 mc (The ant is >cut for 40 meters) >The ATU can tune the V for 10 meters thru 80 meters. I have no idea how >effecient this is...I must say I have my doubts...which brings me to my >question... >Is it better to feed this antenna with say 300ohm transmission line and use >a 4:1 balun at the end of the feed line. And if so why? >I await an answer with baited breath. The feed line will have a high SWR on all bands except 15 meters where the antenna is 3/4 wave length. RG 58 has higher losses than 300 ohm twin lead particularity with a high SWR, so you are better off using the 300 ohm twin lead. The balun can be 1:1 or 4:1, or you can try it without a balun. Try to keep 1/2 of the length of the antenna plus the feed line not equal to a 1/2 wave length on the frequency in use to avoid a high voltage point at the transmitter. Note that the 300 feed line comes in various flavors. Avoid the RS type that is for indoor use only. An alternative to the 300 ohm feed line is a twisted pair of #24 or #26. Use a quality stranded wire PVC or teflon insulated. 73, Chas, W1CG ++++++++++++++++++++++++ Date: Thu, 26 Jul 2001 14:14:24 -0500 From: "Stuart Rohre" Subject: Re: [Elecraft] Feedline for my 40 meter Inverted V Phil and Chris, Sounds like you are a customer for the Bill Orr W7SAI Antenna Handbook, or the ARRL Antenna Book, or others. 300 ohm or other parallel lines have less plastic, thus less dielectric losses than coax. Coaxial cable is very lossy feeding the Vee on bands other than the fundamental. It works but as you suspect not as efficiently as you could with a parallel line for all bans, and a balun. 4:1 would be an approximation, but usually works well transforming to a 50 ohm transmatch. Read up on losses in transmission line chapter of your ARRL Handbook, for a short overview of this. You do have this book at least, right? All hams should have as many antenna books as they can afford if they really want to get out efficiently and have the best results for the investment in antennas. Antennas remain the one area where the ham can easily fashion his own hardware even with no electronics skills, given that he can read and understand practical texts. Those I listed have no extensive math understanding required. Also see the excellent web pages of L. B. Cebik, W4RNL, at http://www.cebik.com. Another good source of antenna info are the web pages of Force 12 Antennas, where a good antenna troubleshooting step by step plan is listed. GL and 72, Stuart K5KVH +++++++++++++++++++ Date: Thu, 26 Jul 2001 14:42:34 -0500 From: "Stuart Rohre" Subject: Re: [Elecraft] Feedline for my 40 meter Inverted V Ron, I don't think you meant your post to sound like it did: Actually, any line with less plastic such as 300 ohm feeder, or 450 ohm window line, (even better) will have lower loss than small coax under the mismatch condition. All parallel lines have lower losses than small coaxial lines under high SWR conditions. That is because the losses in coax are increased by the presence of more dielectric than the parallel ohm line has. The wire on each side of the 300 ohm line has a great angular circumference with only a thin dielectric covering, while the coax center conductor and inner surface of the shield interact through rather thick dielectric as a percentage of the dimensions of the center conductor. Under high SWR, this increases losses. With parallel line, a transmatch with balun can usually match the line to the rig on all bands, and a short extension of the line will take care of any special case. A low loss transmatch should be used. Best of all would be true balanced transmatch with link, removing the need for a balun. Yet, our club has seen no loss with either the Dentron Jr. Tuner, or the Dentron Super Tuner, which has much more rugged components than the small model. On another issue, the 15 m use of the 40m dipole is a long time ham practice, but the actual resonance on 15 will likely be out of the ham band. This could add to the SWR issues using coax. It is widely accepted by the antenna handbook authors, ARRL, Orr, (W6SAI), Cebik, (W4RNL), and others that parallel lines have the lowest losses in multiband use of a single band cut dipole vs. small typical ham coax lines. That has been borne out by our club by experimental use of ladder line feeds on all band use of a dipole. Given a choice of line, I would rather use the 300 ohm window line, (from the Wireman), over the TV types of 300 ohm. Or buy the heavier 450 ohm window line, or for lowest loss, fashion your own air insulated parallel lines. For field dipoles, 300 ohm line is less wieldly to pack than 450 ohm ladder line. Of course for very short distances, say 20 feet or so, even "zip" AC cord CAN be used as others have pointed out in various lists. Another scheme is to take parallel computer flat cable, and peel out a dipole from that, and use two parallel wires left togehter as the feedline for field dipoles. Various sources have calculated higher losses from use of such small wire and plastic dielectrics, but actual users have reported adequate success, certainly at 40m. Losses increase on the higher bands. However, the lossy "zip" cord results the ARRL got some years ago, may not be true of current PVC insulated "zip" cords, or speaker wire. One of these days, someone will run an experiment on losses in modern power cords of the flat, "zip" variety, and in speaker cable of the clear type. I am hopeful they will prove useable for field work. 72, Stuart K5KVH +++++++++++++++++++++ Date: Thu, 26 Jul 2001 14:49:30 -0500 From: "Stuart Rohre" Subject: Re: [Elecraft] Feedline for my 40 meter Inverted V A key issue here is your feeder length, if you add some more you can shift the reactance to a different point and likely find an easier match. Try an 1/8 wave extension of the line at 5.1 MHz. If your dipole is 3/4 wave overall, each fed leg is looking at something longer than a quarter wave, which might be a very reactive match to the center feed. 73, Stuart K5KVH ++++++++++++++++++++ Reply-To: "Ron D'Eau Claire" From: "Ron D'Eau Claire" To: "elecraft" Subject: Re: [Elecraft] Baluns and 450 ohm line Date: Tue, 4 Jun 2002 08:59:06 -0700 Scanning this thread, I think it would be useful to point out something about the use of balanced feedlines to reduce losses, especially with the Elecraft KAT2 and KAT1. There are two ways to reduce feedline losses. One is to keep the SWR at something reasonable. On the lower frequencies, even coaxial line will have reasonably low losses if the SWR is kept to, say 5:1 or less, if it is good quality coax.. Now, you still need an ATU to keep the K2 or K1 transmitter happy. The output filters in the K1 or K2 want to see an SWR of 2:1or less to work properly, but once the line is matched to the transmitter using an ATU, the feedine will work reasonably well even with a moderate SWR. How high? Antenna books will show you charts. The losses vary widely according to the type of coax, the length of coax and the frequency used. The second approach to minimizing feeder losses is to use an inherently low loss feedline such as true open wire feedline. True open wire feedline has almost all "air" insulation. "Air" is the lowest loss dielectric/insulator available. The minimal insulation that does exist to hold the wires the proper distance apart is of very high quality in good line. With this sort of feedline you do not need to care about the SWR. The highest SWR an antenna can produce in a line hundreds of feet long on, say, 10 meters, will not have signfiicant losses. This makes the antenna "design" very easy. You only need a radiator long enough to be efficient (at least 1/4 wavelength at the lowest frequency), high enough to avoid ground losses if it is horizontal, and a good wide-range ATU to handle the matching of the feeders to the transmitter. So-called "window line" and even "twinlead" are compromise "open wire" lines. They have a lot of insulation, so their losses are much higher than true open wire line. Still, in most high-swr applications they will have far lower losses than coax. One other thing is important if you want open wire feedline to act as a feedline instead of a radiator. The currents in the feedline must be balanced. That is, the currents in each wire must be equal and flow in opposite directions. The "balancing" has nothing to do with reducing losses in the line. Its function is to keep the feedline from radiating or picking up energy (such as noise). When the antenna is designed to provide a balanced load and the feedline is fed from a balanced ATU, the r-f field created by one wire is exactly cancelled by the other wire. This prevents radiation and, by the same process, prevents pickup. Any r-f picked up will appear in phase on both wires - what engineers call "common mode" currents. These "common mode" currents get cancelled out at the balanced circuit at the transmitter end of the feeder. That leaves only the "out of phase" currents - the signals coming down the feeders from the antenna - to be passed on to the receiver input. Balance is not always critical or even desirable! Some popular antennas such as an "off center fed" doublet and "Carolina Windoms" that use open wire line assume that the "feedline" is part of the radiating antenna. Lack of feedline balance does not affect their efficiency as long as they are installed to avoid losses from objects near the feedlines. Some of these designs use a balun at the end of the open wire feeder section. The purpose of the balun is to avoid currents flowing on the outside of a length of coax connecting it with the rig inside the shack. The unbalanced load on the feeders at the antenna end will ensure that the feeders radiate (and pick up r-f) in spite of the balun. When two coaxial lines are used in parallel as a sort of "shielded" balanced feeline, the fact that the line may be balanced does nothing to reduce the losses. The paralleling of two coaxial lines in a balanced system is done to raise the feeder impedance which can reduce the SWR in a certain antenna designs, as Stuart pointed out. Managing the SWR in such a system is still critical if you want low losses. The r-f field around each wire is still trapped in the dielectric between the center conductor and the shield in each coaxal line. That's where the losses occur, but it also makes the balance less critical since there won't be any radiation or pickup from the line because of the shielding. With true open wire line, these concerns about keeping a moderate SWR to avoid excessive losses do not exist. The KAT1 or KAT2 ATU's in the Elecraft rigs have an excellent matching range, and are capable of handling *almost* any impedance a true open-wire feedline is likely to present. But there is one concern for operators using the KAT1 or KAT2 ATUs to be concerned with. That is providing balanced feed if they do not want the feed line to radiate. The common way to do this using a "single ended" ATU like the K-rigs have is to put a balun at the output. Baluns get unpredictable with very high SWR's. More often than not they seem to work fine, but there is no guarantee that common torodial baluns will work if your feedine impedance at the transmitter is extremely low or high. When using open wire line in this manner, you don't care what the impedance of the line is, nor is there an obvious link between feedline impedance and the impedance presented to the rig in practical designs. Back in the 1930's when open wire lines were the most common sort of feeders, few Hams had any way of measuring SWR nor did they care. They discovered that an efficient antenna system sometimes presented an impedance to their ATU's that made adjustment very touchy. Sometimes they could not affect a match. The more efficient the antenna, the worst this situation was at certain frequencies. (Lossy antennas seldom have large changes in SWR. A good dummy load presents a 1:1 SWR across the spectrum, for example). If you peruse the manuals for center fed doublet antennas using good open wire feedlines, you will find tables showing combinations of antenna plus feedline lengths to avoid. That's why. Those are the lengths likely to produce such extreme impedances at the ATU that it may be difficult to find a match. The same is true today. The impedance presented to the ATU will be a function of the frequency, feedline length and antenna system and will vary over a very wide range - perhaps from a few ohms to several thousand ohms. You may find that it is difficult to find a match on some bands even using the KAT1 or KAT2. In that case, it may be necessary to change the length of the feedline (or antenna) a little. It's a bit more difficult, nowadays, because of all the new bands we have, but a compromise that allows one to operate on all bands can be found... usually . Ron AC7AC K2 # 1289 +++++++++++++++++++++ From: k6se at juno.com To: kc4kgu at ENTERZONE.NET, elecraft at mailman.qth.net Date: Tue, 4 Jun 2002 07:59:01 -0700 Subject: Re: [Elecraft] Baluns and 450 ohm line John, KC4KGU wrote: "I currently feed my 75m doublet with twin pieces of RG8X. This presents a line impedence of about 100 ohms. Prior to feeding with twin pieces of coax, I cut the doublet for resonance at 3.875Mhz. It showed a 1:1 match at 50 Ohms according to the MFJ-269 analyzer." Now, since the MFJ-269 is designed to give readings based on 50Ohms, what should I be looking for when I check one of my antennas that is being fed with twin coax with a 100 Ohm feedline impedence?" ========== I'm curious to know why you would want to feed an antenna that has a perfect match at 50 ohms with a 100-ohm feedline. If the antenna input Z is truly 50 ohms, then if the feedline is a multiple of 1/2-wave long electrically, the shack end will "see" 50 ohms Z. Otherwise, an Autek Research VA1 RX Vector Analyst may be used instead of the MFJ-269 becaise the VA1 is not limited to 50-ohm lines. You can select SWR readout for over 10 common feedline impedances from 25 to 450 ohms with the VA1. I'm not associated with Autek, but I own a VA1 and find it more useful and accurate than my MFJ-259. The Web site for the VA1 is http://www.autekresearch.com/va1.htm . 73, de Earl, K6SE 67 countries with K2 #2622 and counting..... +++++++++++++++++++++ Subject: Re: [Elecraft] Baluns and 450 ohm line Date: Tue, 4 Jun 2002 13:27:13 -0400 To: "Ron D'Eau Claire" , "elecraft" From: Bill Coleman On 6/4/02 11:59 AM, Ron D'Eau Claire at rondec at easystreet.com wrote: >The second approach to minimizing feeder losses is to use an inherently low >loss feedline such as true open wire feedline. True open wire feedline has >almost all "air" insulation. "Air" is the lowest loss dielectric/insulator >available. The minimal insulation that does exist to hold the wires the >proper distance apart is of very high quality in good line. Uh, Ron, I'd like to point out that at HF, the vast majority of the loss in open wire or coaxial lines is resistive in nature. The reason that open wire has such low loss is that a) its conductors are large, usually 14 guage or larger, and b) its impedance is high, which leads to very small I^2R losses for the matched condition. All feedlines exhibit higher losses when feeding a load that does not match their characteristic impedance. These losses are due to reflections traversing the line. >With this sort >of feedline you do not need to care about the SWR. The highest SWR an >antenna can produce in a line hundreds of feet long on, say, 10 meters, will >not have signfiicant losses. Actually, SWR does matter in open wire lines. Particularly if the SWR is quite high, the losses due to reflections can mount up considerably. There are cases where an antenna that would closely match a quality 50 ohm coaxial cable will show more loss when connected directly to open wire line, due to the high SWR. Traversing a 12 guage conductor ONCE is going to have less loss than traversing it 20 or more times. >So-called "window line" and even "twinlead" are compromise "open wire" >lines. They have a lot of insulation, so their losses are much higher than >true open wire line. Still, in most high-swr applications they will have far >lower losses than coax. These feedlines have higher losses usually because a) they use smaller conductors than open wire, and b) they have lower characteristic impedances, which tends to increase the I^2R losses for the matched condition. >The r-f field around each wire is still >trapped in the dielectric between the center conductor and the shield in >each coaxal line. That's where the losses occur, but it also makes the >balance less critical since there won't be any radiation or pickup from the >line because of the shielding. Again, most of the losses in HF are resistive. Coax is more lossy in part because the center conductor is rather small. >With true open wire line, these concerns about keeping a moderate SWR to >avoid excessive losses do not exist. Actually, they do exist. However, the very low matched loss of open wire lines tends to keep the losses due to reflections down until the SWR reaches very high levels. Bill Coleman, AA4LR, PP-ASEL Mail: aa4lr at arrl.net Quote: "Not within a thousand years will man ever fly!" -- Wilbur Wright, 1901 +++++++++++++++++++++ Reply-To: "Ron D'Eau Claire" From: "Ron D'Eau Claire" To: "elecraft" Subject: Re: [Elecraft] Baluns and 450 ohm line Date: Tue, 4 Jun 2002 11:15:30 -0700 Bill wrote: > Uh, Ron, I'd like to point out that at HF, the vast majority of the loss > in open wire or coaxial lines is resistive in nature. I disagree, based on what I read in many books on antenna and transmission line design. Resistive losses are part of the loss, but not the "majority" in coaxial line especially. To quote just one reference most Hams have, the ARRL Antenna Book, "..the loss characteristics of coaxial cable depend largely on the dielectric material between the conductors..." > All feedlines exhibit higher losses when feeding a load that does not > match their characteristic impedance. These losses are due to reflections > traversing the line. Right. But the increase in losses is very small when the line has low inherent losses. Because the losses of good low-loss open wire line increase so slowly as the SWR goes up, the total loss of good open wire line at high SWR's is typically less than good coax line operated at an SWR of 1:1 For example, the SWR of 100 feet of open wire line at an SWR of 20:1 at 30 MHz will be 0.9 dB total. Good quality RG-8 looking into that same 20:1 SWR will show a loss of over 5 dB. Even if the RG-8 is perfectly matched for minimum loss, 100 feet of it will show a greater loss than the open wire line at about 1.0 dB at 30 MHz.. > The reason that open wire has such low loss is that a) its conductors are > large, usually 14 guage or larger, and b) its impedance is high, which > leads to very small I^2R losses for the matched condition. The design information for open wire lines that I have (again, one example is the ARRL Antenna Book) do not even factor in the size of the wire used in evaluating line loss - only the dielectric qualities of the insulation and the amount of insulation used to hold the wires. The conductor sizes do affect the "characteristic impedance" of the line. That is, the impedance of the circuit it must be used in if the SWR is to be 1:1. When we speak of "450-ohm ladder line" or "300 ohm twinlead" we are speaking of the "characteristic impedance" of the line. That has little real relationship to the impedance of the line in a real antenna. The impedance on either of those lines can vary from a fraction of an ohm to over a thousand ohms when connected to an antenna where the impedance of the antenna does not match the "characteristic impedance" of the feed line. It is the actual impedance extremes found in the system in which the feed line is used, not the characteristic impedance of the feed line, that will determine the resistive losses. Ron AC7AC K2 # 1289 +++++++++++++++++++++ Date: Tue, 4 Jun 2002 18:08:17 -0400 (EDT) From: kc4kgu at ENTERZONE.NET To: N2EY at aol.com Cc: elecraft at mailman.qth.net Subject: Re: [Elecraft] Baluns and 450 ohm line On Tue, 4 Jun 2002 N2EY at aol.com wrote: > In a message dated 6/4/02 2:53:58 AM Eastern Daylight Time, > kc4kgu at ENTERZONE.NET writes: > > > I currently feed my > > 75m doublet with twin pieces of RG8X. This presents a line impedence of > > about 100 ohms. Prior to feeding with twin pieces of coax, I cut the > > doublet for resonance at 3.875Mhz. It showed a 1:1 match at 50 Ohms > > according to the MFJ-269 analyzer. > > I'm a little confused with your setup. > > Are there two pieces of RG-8X "side by side", with the center conductors > going to the feedpoint of the dipole? > > Is there a balun in the setup somewhere? No. No balun. The two center conductors are used for the feedline and the shield floats at the antenna and is grounded at the shack. > > > Now, since the MFJ-269 is designed to give readings based on 50Ohms, what > > should I be looking for when I check one of my antennas that is being fed > > with twin coax with a 100 Ohm feedline impedence? > > That depends entirely on the length of the coax. A basic example: > > If the electrical coax length (don't forget velocity factor) is an integral > multiple of a half wavelength, the impedance at the feedpoint will be > "repeated" at the other end - regardless of the transmission line used! > > 73 de Jim, N2EY 1/2 wavelength of coax including velocity factor. 73 de John - KC4KGU K2/100 #2490 +++++++++++++++++++++ From: "Stuart Rohre" To: Cc: "elecraft" Subject: Re: [Elecraft] Baluns and 450 ohm line Date: Tue, 4 Jun 2002 18:09:13 -0500 John, 50 ohm based measuring instrumentation should have enough available current drive, at its low impedance, to accurately drive, and allow measurement, of the higher impedance cases. The principle here is that a higher impedance will not load down the instrumentation, given 50 ohm devices with sufficient current sources. However, remember that unless you have exactly a half wave length line, if the antenna is some impedance other than 100 ohms, it may not show its true Z thru a random length of 100 ohm line. In the exact half wave length at one frequency case, the 100 ohm line becomes transparent as to impedance, and does not affect the total impedance, allowing you to measure the antenna impedance back at the shack end of the half wave line. You would look for the minimum, or dip, in the measured SWR on the MFJ. The actual value of impedance reported will be perhaps something different than antenna Z, for random length feedline, but the dip will be the resonant point of the antenna. A resonant antenna at optimum height will measure closer to the 72 ohms of free space, not 50 ohms anyway. If you had 50 ohms before, it was a fortuitous happening of the height of the antenna, and its influence with nearby conductors or other near field couplings. Don't worry about the exact antenna impedance if you are using a transmatch anyway to move over the band. On 80m, no single wire antenna, that is high efficiency, will hold a 1:1 SWR over the band without a tuner. If your antenna is so low Q as to have low SWR over most of the band, look for losses, or other problems. You should have to use a transmatch to move around the 80m band with most ham wire antennas. They are too high Q to load well at all 80m band frequencies. But this is OK, you can accept some SWR up to the limits of your rig, or use your transmatch. The efficiency of the antenna will not be lessened by the SWR, all the reflected power is eventually radiated. The losses come about in low impedance feedlines having higher loss inherently, and the power having to transverse the line multiple times until eventually radiated. Although there are losses in the higher impedance parallel lines, it is very much less than coax loss, when open wire or even insulated ladder lines are used. I do not think there is any advantage to using twin coax to feed an antenna, when you consider the downsides of extra weight on the center insulator as compared to most parallel lines and the extra dielectric in the coax.. The use of twin coax is often thought to lessen RF from the line, but certain lengths of feeder, or failure to have balanced currents at the feed point may introduce RF onto the outside of the shields and you are right back to having a radiating unbalanced condition. Beware of the line, if choosing a particular line broadened your SWR curve flat portion! Yes, the rig will load it, but the RF is not going where it is needed. The wonderful thing about ham radio, is under the right propagation conditions, you can have a poor antenna, balanced in construction, but unbalanced by feeding by single coax and no balun, and it will work out just fine. I doubt you could measure the difference on an S meter between the ideal feed and whatever you have at hand. It is the other effects of unbalance that might cause problems, (line radiation, etc.) But, an antenna is a system that consists not only of the feeder, but also the antenna and its materials, and the local RF ground character, elevation, and anything close enough to couple to the antenna. You must have knowledge of all factors affecting your installation to get it operating to top efficiency. -Stuart K5KVH +++++++++++++++++++++ From: "Stuart Rohre" To: "Ron D'Eau Claire" , "elecraft" Subject: Re: [Elecraft] Baluns and 450 ohm line Date: Tue, 4 Jun 2002 18:35:31 -0500 Well put Ron! Your expansion of the discussion on balance reminded me to point out that besides the unbalance of doublets with unequal legs such as the Windom, the equal legged doublet can introduce imbalance (unequal feedline currents). How does this happen? In some locations, you might have run one leg of the doublet over a roof of a building, while the other leg is in the clear, with nothing under it. Or, one leg might run closer to some overhead electrical wires than the other leg. There are various such influences that can upset the line current equality you desire when using parallel feeders. At least in this area, I find RF ammeters still available at ham swaps, and a pair of matched models is a good investment, if one wants to seriously explore the issues in using parallel lines. They were common tune up aids in the single wire military aircraft radio outputs. The idea is to upset both sides of the parallel line equally, by inserting two ammeters one in each leg of the feeder, and compare the current indications. It helps to mount them in a non conducting box, and otherwise avoid introducing losses and non symmetry to the measurement setup. 72, Stuart K5KVH +++++++++++++++++ Date: Tue, 04 Jun 2002 20:17:57 -0400 To: "Ron D'Eau Claire" From: Charles Greene Subject: Re: [Elecraft] Baluns and 450 ohm line Cc: "elecraft" At 11:15 AM 6/4/2002 -0700, Ron D'Eau Claire wrote: >Bill wrote: > > > Uh, Ron, I'd like to point out that at HF, the vast majority of the loss > > in open wire or coaxial lines is resistive in nature. I made have some home made transmission line using #26 silver plated wire. It's characteristic impedance is about 200 ohms. We planned to use 37' for field day. KC1SD calculated that it had a RF resistance (different from DC resistance due to skin effect) of 5 ohms for 74' at 7 Mhz. He said he used Terman's. That's not much loss, but the dielectric losses are low too, so it is possible for that piece of transmission line, the ohmic losses might be more than the dielectric losses. Not so for coax where the wires are larger giving less RF resistance and more dielectric loss. Anyway, the RF resistance loss can be estimated and subtracted from the total loss which is listed in the specifications for a particular coax. 73, Chas, W1CG ++++++++++++++++++ Reply-To: "Don Wilhelm" From: "Don Wilhelm" To: , , Subject: Re: [Elecraft] Baluns and 450 ohm line Date: Tue, 4 Jun 2002 23:21:58 -0400 Folks, If I recall the antenna in question was a dipole. I just constructed a 40 meter dipole a few weeks ago and cut it to resonance - that is I cut it for no reactance shown on my MFJ 259 - and at that point, the impedance was 73+j0!!! So I fed it with 75 ohm coax for the lowest loss on the feedline - coiled up 15 feet of the coax on a 4 inch form and placed that balun at the feedpoint of the antenna. It works great, and yes, I have to suffer with a 1.5 SWR at the transmitter or use the tuner on it to get to 50 ohms. BTW, 72 ohms resistive is the normal impedance for a proper dipole at a decent height. What is the point you ask - well, I am wondering about the obsession with 50 ohms. When an antenna that is inherently has a 72 ohm feedpoint is pruned to show 50 ohms on the meter, it will have reactance included - then the length of the feedline becomes critical, the SWR is not zero even on a 50 ohm feedline, and other strange things rear their ugly heads. To my mind, it is better to either match the line to the antenna at the feedpoint, or use an L network to match at the feedpoint, or just bite the bullet and feed with open wire line. Just because the transmitter wants to look at 50 ohms, pruning the antenna to get that impedance is not the only important thing - making the reactive component is more critical - and is the true indicator of resonance of either tuned circuits or antennas. Also, the parallel coax feedline is useful for maintaining balance, but the matched line loss is the same as the matched loss for a single coax feedline alone - which can be very high if that antenna is used for multiband work. I often use parallel 93 ohm coax to bring a parallel feedline into the shack, but I use only a short length and connect it directly to the 450 ohm ladder line despite the impedance bump it creates in the line. 73, Don W3FPR ++++++++++++++++++++ Date: Wed, 5 Jun 2002 12:59:34 -0400 (EDT) From: kc4kgu at ENTERZONE.NET To: Dan Barker Cc: Elecraft Subject: Re: FW: [Elecraft] Baluns and 450 ohm line On Wed, 5 Jun 2002, Dan Barker wrote: > Wow! That's quite a bit of wire - weight - money - wind load - failure > points - up there! > > What's the point? Can you please explain why an open wire fed 160m > loop and KAT2 would do worse? Maybe if each loop were lower with > higher freq (to control the takeoff angle?). Beats the poop out of me. > > Dan / WG4S / K2 #2456 > > I have since built full-wave loops for 160m, 80m, 40m, 30m, 20m, > 17m, 15m and 10m. The answers are: (1) Pattern and angle of radiation. When you start moving UP in frequency from the design frequency, the antenna will begin to exhibit more gain, flattening the nice sphere radiation pattern you get with a full-wave loop into what sometimes looks like a clover-leaf and what other times resembles a bug that hit a winshield at 75mph with little or NO energy going to high angle radiation and most of it down in the 25-40deg takeoff angles. This is great if you want to work 10m DX with your 160m loop. If you want to talk to the guy 300mi away, you'll bounce right over him though. He'll never hear you. With an antenna for each band of interest, I have the choice. I can use the one cut for the band and get the nice sphere pattern, OR I can use one of the antennas designed for a lower frequency and achieve gain and low angle radiation. The best of both worlds and it all fits in a nice square 135ft per side. (2) It is seriously not that much wire, weight, wind load, or failure points. The ENTIRE antenna system has less wind load than your _average_ amateur tower. There is about 1250ft of wire used for the antennas. They weigh maybe 25 lbs or so, all together. You call them failure points, I call them REDUNDANT systems. If something happens to one of the antennas, I can use one of the others. It may not be as efficient, but, it WILL tune up. If a person only has one antenna and something happens to them, I suppose that they can load up their dummy-load but, I somehow don't think it's going to have the radiation efficiency of one of my loops. As for it being lots of money involved, I have less than $300 or so invested in this antenna system and that includes the remote coax switch. Wire is cheap at Home Depot. Coax is cheap when you buy it in large bulk. Remote coax switches are cheap on eBay. The only thing I've invested a lot of in this project is my time. It is something I enjoy doing. I have been planning an elaborate antenna system ever since the first snow hit the ground last winter. I made it through winter with a 75m dipole, coax fed. I promised myself that next winter would be different though. 73 de John - KC4KGU K2/100 #2490 +++++++++++++++++ Reply-To: "Ron D'Eau Claire" From: "Ron D'Eau Claire" To: "Elecraft List" Date: Sat, 8 Jun 2002 00:27:34 -0700 Subject: [Elecraft] A Correction About Feedlines... A while back I made a post about using K-rigs with their ATU's with open wire vs. coaxial feedlines. I stated that open wire feedlines were more efficient than coaxial cables at high SWR's because the dielectric losses were higher in coaxial lines. A couple of readers observed that they thought it was the resistive losses, not dielectric losses, that were the cause. I replied quoting design handbooks, principally ARRL Antenna books written 20 years ago, as saying the losses were caused mostly by the dielectric, or insulation, used in the feedlines. After a number of off-reflector exchanges with readers, it is clear that the sources I was quoting are no longer claiming that to be true. In particular, the more recent ARRL antenna books say. "Dielectric losses are generally negligible below UHF frequencies." What has changed? Perhaps insulators are better these days. Perhaps a long-held opinion was overturned by better test data. I don't know. My thanks to those who questioned my statement. You started an interesting dialogue exploring the subject and looking for references. In particular, thanks to Lou, W7DZN who found the latest quote in new ARRL book, "Simple and Fun Antennas for Hams." In the section on transmission lines. What remains true is that well-constructed open wire feedlines are still more efficient than coaxial lines, and when the SWR's get up into the range of 10:1 or 20:1 and more, the superiority of open wire lines is substantial. Ron AC7AC K2 # 1289 ++++++++++++++++++