RADIOACTIVITIES Newsletter of the Argonne Amateur Radio Club
Volume XLVI, Number 4	April, 2005

02		N9ASC		Bill		South Elgin, IL
03		KC9AZJ		Thomas		La Grange IL
17		N9MBR		Bob		Channahon, IL
19		WB9WOC		Jerry		Kankakee, IL
19		KA9PMZ		Robert		Lansing, IL
22		N9SXG		Georgiann		Glen Ellyn, IL
27		KC9EUX		Renae		New Lenox, IL

The unique part of this W3HJR article bringing fanning of the ends using 2 wires or coaxes of the same length separated by 0.03 waves. Switching to coax changes the velocity factor to 0.66 for most all cable of this type; formula becomes 492 * 0.66 / F MHz cut for 80 meters center at 3.750 makes the coax folded part 86.6’ with 2 other coax sections of 19.2’ each and using the fanning technique would add another pair of coax or wire sections 19.2’ separated 0.03 waves apart forming an angle. It may be tough to keep it up around the Windy City. This aerial shows SWR at band edges 3.5 and 4.0 MHz 2 to 1 if your resonant frequency is 3.750 MHz.

The advantage to the folded dipole is bigger bandwidth, ease of tuning, less sensitive to placement effecting SWR, and receiver noise. Readers in June 1973 were treated with an article using coax over twin lead by W2EEY. Type of coax used 70 or 50 ohms with number 12 wire used at the coax ends. The overall length of coax and wire is by formula 462 / F MHz and the center coax section formula is 326 / F MHZ. Now we cut our coax from formula in the center, cutting 1 inch of vinyl covering then cutting shield in center folding back making two leads. Don’t cut the center conductor with its dielectric. Here the feed line will be connected later to the two shields. The ends of the coax should be shorted shield to center conductor also attaching one wire section to each end.

Each coaxial section forms a shorted stub across the dipole terminals less than ¼ wave long produces an inductive reactance. This reactance of the stub changes with frequency as does the antenna terminals as the electrical length of the aerial changes. Thus, reactances cancel each other and impedance looks mostly the same. The resistive component of the impedance varies slower than the reactive component of the impedance around the frequency of the coaxial dipole. How can you hold this antenna together when you raise it. The answer is a 3 by 3 inch Plexiglas or bakelite or other insulating material using U clamps on each of the 3 coaxial line are held it in place. Once constructed, waterproof with bathtub caulking sealing everything like coax ends with attached wires.

Another article appeared in May 1981 73 Magazine by a XYL WA9BBX. She writes about a solution to landlord blues taking her coaxial aerial indoors. More than one antenna can be put up in the same area, but they are to run at angles to each other rather than being in parallel. The inactive aerial may absorb signal from the active aerial dropping the true signal output. The vinyl jacket reduces static charge build up. Maximum power can be used with RG8U or RG58A/U. She gave dimensions for 5 bands and she did it this way. For 40 meters used 63’ of cable removed 1 inch of vinyl jacket at the center of the aerial; inch each side of center and cut the shield all the way around the coax and don’t cut the dielectric or center conductor. Form two leads with the shields; here is the feed point. From the center of the feed point, measure out 16’ 9“ as well on the other side of the center cut coax at these two points remove 1 inch gap. Now on each leg twist shield and center conductor together solder. This forms the matching section and balun. Measure coax 2 pieces are needed 14’ 9“. Remove 1 inch vinyl jacket from them for ends then fold back shields cut the 4 dielectrics only center conductors and shields twist and solder then. Solder one 14-foot section to one end of matching section and other 14-foot section to the opposite side of the matching section. Waterproof all solder joints except feed point. Take feed line & remove 1 inch or more remove vinyl jacket cut dielectric, exposing center conductor, form two leads solder each lead to one of the antenna center shields and waterproof even if these are indoor aerials its best to do it. Erecting across an attic, roof, living room, bedroom, or other places you find and wrap cable ties around aerial at intervals staple to ceiling but not through antenna. Use it as a drooping dipole or inverted U. Get as much as you can into a straight line. Keep it away from metal objects and wall wiring. Use SWR Bridge to get a reading with low power move from low frequency to high. Find lowest SWR of trim each end of aerial one inch at a time until you get low SWR. Waterproof ends. Her results over 2 years working ten, fifteen, twenty, and forty meters yielded worked all continents, worked all states, and DXCC 121 countries.

We come to the ¼ wave folded dipole with its 6,000-ohm feed point impedance. We cure that problem with a ¼ wave matching line transformer. Use 670-ohm open wire line for 70-ohm coax or 385 ohm or 450 open line ¼ matching line to match 50-ohm coax resulting in low SWR.

A favored antenna in the 1950’s was the ¼ wave Multee. This popular aerial uses 300 ohm folded section and for ¼ l down line matching section just more 300 ohm twin lead and hooks up to 50 ohm coax right by mother earth where the cable has 4 radials to the shield side of coax and one side of the 300 ohm down lead. Major feature of this Multee is its two-band operation in restricted space. On lower frequency it function as a top loaded vertical then on the higher band it is a ¼ wave fed into a matching transformer from a 50-ohm line. Radials are used to get a ground return and decouple balance unbalanced feeding effect with coax cable.

For better favorable angle on a tower, stack the lower frequency mono bander high up and the higher frequency mono bander lower for the right below 30 degree DX angle. However, there is the exception note that I worked FB8W, FB8X, and FB8Z these are very remote French Islands way beyond Australia. The distance from Chicago is 10,441 miles, 11,220 miles, and 11,642 miles. During good sunspot cycle activity in early 1980’s with my FT101B, 150+ watts, no linear on 20 meters early 6am one morning using my 130’ long wire. My reports on the YL net system was 5 by 5, 5 by 4, and 4 by 4 respectfully. The wire was up 30’. How about that? It can be done the hard way with simple equipment and low aerials. These were phone contacts. Who says coax is a poor radiator. “Tunnel Radio” reported in World Radio Jan 2005 by Kurt Sterba claims coax will radiate if small slits are made in the shield all along the line. This system has an aerial outside the tunnel these signals are amplified and sent through the tunnel wide band signals. This system works on AM and 47MHz to 860 MHz. The cable is placed along the ceiling of the tunnel. Similar systems are used in miner through out the world and in railroad tunnels.

Here is one weird antenna take note of KF4BWG a 146MHz tree aerial uses the sap of a tree for an antenna. Drive a nail into a tree until you get 1 to 1 SWR using an antenna analyzer. Hey! You can see this sap on the internet or the photos thereof. Try www.qsl.net/KF4BWG. Well, who will be the next sap to try this one? Say how about a 100’ tree for 160 meters now that’s got me thinking again.

Speaking of low band verticals, 40 meters is best when the wave path is all or nearly all in darkness for DXing, what you need is low angle of radiation confined to 15 or 20 degrees and a good receiver. The optimum for 2500 miles is below 15 degrees and can reach 5 degrees. The best antenna used in the 1920’s by the old timers was the “30-30” that is the 30’ vertical and 30’ horizontal. The vertical element was topped off with a toilet copper ball and the horizontal leg was a counterpoise a few feet off the ground. It resembled a direct fed Brown Ground Plane but with one radial. It was a good 360 degree DX performer and common to work the world with 25 to 50 watts input in a self excited oscillator. Vertical is the way to go for low frequencies.

Enter the bobtail; discovered in the 30’s and used into the 40’s and to this day revered as a special type of antenna. Not long ago I overheard a group extolling the best antennas only if they had the room for them. They being real estate poor. The best for 80 and 40 meters is this bobtail so I remember reading about it so I did some research. The half power beam width is 60 degrees directivity power over a 1 element is about 5db and the power gain due to horizontal directivity is like an increase in transmit power almost 4 times. The bobtail should for forty meters be 40 to 45’ high. Feeding the center vertical element puts the current loop at the top of the horizontal section and it is voltage fed near ground at the bottom. The impedance is high and bandwidth covers the whole band. The two outside elements are pruned to frequency and each are made the same length. Therefore, we have 3 vertical sections and spacing of wave either side of center. The two outside vertical elements are 33’ long and distance to center from each is 66 feet with the non-critical center vertical 30 to 36’ long. For 80 meters, multiply these dimensions by 1.82.

Along comes W8HXR who wrote in 1980 73 Magazine using the voltage feed idea for 20 meters, but W1XU wrote to him and found it easier to use 52 ohm coax at the height current point at the top of the center element. N6AUV called W8HXR said he too was using current feed at the top and it worked better. Then he heard from W9ZSI who is putting up another 20 bobtail and phase the pair with quarter wave spacing driving both aerials, and W8YFB wrote he was using voltage feed with open ladder line and working 80, 20, 15, 10, and tied feeders together for a bobtail on forty.

How to current feed a bobtail? The center of coax to top of center vertical of the array then braid or shield goes to the flat top horizontal portion out to the outer verticals. You may feed just one of the outside vertical for convenience and this will favor one direction over another in pattern but will still work very well. This is a quiet aerial and long range. It’s best when path is over 2500 miles. W8HXR is driving his bobtail from top of one end vertical favoring the west and this method is related to a full wave long wire thus this pattern. Now if you tie the shield of the 50/75 ohm coax to the vertical and the hot center conductor to the flat horizontal section the destruction of the vertical phasing will take place.

For 20 meters the flat horizontal sections are not too critical and are 33 or 34’ long. The verticals length is 16’ 6” for SWR of 1 to 1. The horizontal section affects the pattern. Only tune the verticals if the SWR isn’t 2 to 1 or more. It will work better laying a roll of coated green fencing under the antenna - no connection to the aerial, it is just a reflecting surface.

Now in July 81 in 73 Magazine W6RCL gives his rendition of a 15 meter bobtail it’s different. He uses sections of tubing and on top of his home flat roof 46’ long. The 3 verticals are spaced 23’ apart each element 11 ½ feet long. He cut wire one wave long (46’) and tied the wire to the two outside verticals. He ran the coax to the middle vertical (not grounded) the transmission line coax center conductor tied to the bottom of the center vertical and the coax shield to the horizontal phasing wire and at this point ran 4 radials ¼ wave long. The bobtail radiates broadside. The 4 radials lowered SWR.