++++++++++++++++ Date: Sun, 29 Sep 2002 11:13:38 -0700 To: Charles Greene From: Eric Manning Cc: elecraft at mailman.qth.net Subject: [Elecraft] lo-noise verticals Chas I have a centre-fed, shortened [by centre loading coils and capacitance stubs on the ends], halfwave vertical [ a Force 12 Sigma 5 centre-fed dipole stood on its ear] on the stern of my boat on salt water, and I used a bobtail [2 quarter-wave verticals fed in-phase using a full-wave centre-fed dipole as the phasing line] this summer on the edge of beautiful Lake Huron, all on 20m. Both are noticeably lower noise than my horizontal dipoles. [I converted a 7 MHz dipole into the 14 MHz bobtail by hanging quarterwave wires from its ends, so I had a dipole to compare it to.] K4XX the bobtail guy [July 02 QST] says that a lot of noise arrives at high angles of incidence which is why the bobtail is lo-noise. The bobtail is lo-angle and the Sigma 5 is too - it can be as low as 10 degrees over salt water according to the designer, which mine is [being 3 feet above the Pacific Ocean!] Also, some of the noise has got to be horizontally polarized. I had very gd DX results this summer from both with the K2 -kazakhstan, croatia & malta. [speech] Anybody interested in portable or maritime mobile verticals should get or build a centre-fed halfwave vertical, full length or shortened, and forget all that stuff about quarter-wave verticals using insulated backstays, and trying to get an rf ground with the keel IMO. The centre-fed vertical dipole removes much [all?] of the need to get a near-perfect rf ground. It does not rely on the ground to provide one half of the halfwave dipole as does the traditional quarterwave vertical at ground level. As for fixed use, I have had good luck with quarterwave verticals at least a halfwave in the air with several radials, also up in the air, for rf ground, but terrible results from quarterwave verticals with their bases on the ground. I just couldn't get a good enough rf ground from the earth. [end speech] I'm going to use the Sigma 5 portable S2 if the Bangladesh Telecomms Regulatory Commission will issue a letter of permission to operate [fingers crossed, touch wood]. If so, we'll see how it does with the K2 for land portable. eric on the other coast -- Original Message -- Date: Sat, 28 Sep 2002 14:11:21 -0400 To: elecraft at mailman.qth.net From: Charles Greene Subject: [Elecraft] Portable antenna Hi, A week or so ago, Wayne was asking about a low noise and I recommended a vertical. Here's my latest finding: I made up a 20 meter vertical for portable use and installed it on my sea wall for test. It is full size, and I used some junk 1" aluminum tubing I had in my attic. It has two wire radials elevated 1' above the ground. I made the radiator slightly short and the radials about 2' long to bring up the feed impedance to 50 ohms and also the impedance of the radials so one of them does not hog the current which it is inclined to do if all radials are at zero ohms. The first contact I made was day before yesterday, and I got a S9+20 report from the Fiji islands, running 30 watts on PSK31. Then I ran some tests on it. It is located about 50' farther away from the house than my 6BTV and G5RV. Using Spectrogram, I determined the noise floor was 10 dB (voltage) lower than the 6BTV and 16 DB lower than the G5RV. The received signals were about the same as the other two, with a slight edge, 1 to 2 DB stronger. On comparative reports I have received on transmit, it is running "slightly" to 1 S unit more than the other two. On DX, it is at the edge and 8' above sea level at high tide of Narragansett Bay, 4 miles wide at this point. On the other two antenna, my 6BTV has 22 radials of 480' of wire, and the G5RV is 30 ft high and about 50' higher than the sea level of the bay at high tide.. Both seem to work well. I knew the G5 was noisier than the 6BTV, but that usually does not come into play unless the signals are so weak they are in the noise level. I really can't explain why the vertical is so low noise (my house isn't that noisy), and I would have guessed it would have performed a little less well than the doublet and the vertical antenna with a fair number of radials. I can understand why it is better on the long haul stuff, as it has nothing between it and the frezonal zone a mile or two away. I would appreciate any comments. 73, Chas, W1CG K2 #462 ++++++++++++++++++ Date: Tue, 01 Oct 2002 07:34:33 -0400 To: Bill Coleman From: Charles Greene Subject: Re: [Elecraft] Portable antenna Cc: Bill and All, Thank you for your comments. See my comments below: At 09:33 PM 9/30/2002 -0400, Bill Coleman wrote: >On 9/30/02 10:18 AM, Charles Greene at W1CG at QSL.net wrote: > > >Three points. 1. The vertical performs a little better than both the > >Hustler 6BTV with 22 radials and the G5RV on both receive and transmit > >which it would not do if it were that much worse (5 and 8 dB respectively) > >that those two on receiving noise. > >I have no answer to this other than to ask exactly how you determined >that the antennas "perform" the same? I used Spectrogram, measured a received PSK31 signal using the cursor to which is accurate to 1 dB to measure the amplitude of the received signal, then quickly switched antennas and measured it again. The readings were nearly always within 3 dB of each other, and all cases, the new vertical was receiving better. The biggest difference between the antenna is the noise floor. On Spectrogram the noise floor was -68 dB for the new antenna. For the Hustler 6BTV, it was -58 dB, and for the G5RV, it was -52 dB. The readings are dependent on the gain settings of the receiver and the sound card, and were made at 900 hz. Spectrogram reads in voltage dB which uses the 20 log function. For transmit, I made contact and asked the receiving station to give me a signal report on two different antenna. A few times I used low power and switched the antennas hot. The signal reports I received from other stations were higher when using the new antenna in all cases. > > 2. There are many studies that indicate > >that using more than two (or four for symmetry) radials does not improve > >the performance of elevated radial verticals. > >This is true when the radials are elevated 1/2 wavelength above the >ground. Near the ground, things get more complex. Of course, we're >talking about 1/4 wave radials (or longer), not two foot pieces of wire >as were suggested in the original posting. I'm sorry if I led you to believe the two radials were two foot long pieces of wire. I ran tests on radials of #12 wire, 15' long with a radiator of 18' varying the length of both the radiator and radials some, radials 17'6" to 20' with a radiator of 16' varying the length of both some, and finally settled on radials which are 17' 1", and a radiator of 16'6". The radiator is made from 1 1/4" telescoping tubing, which I could easily vary the length. I ran tests both with an earth ground connected and not connected. (Results seem a little more consistent with the earth ground disconnected). The feed line is perpendicular to the radials for about 1/2 wavelength, and has a 1:1 bead balun at the antenna to keep the feed line from acting as an unintended radial. Tests I ran were measuring SWR, Z and R+/-jX across the 20 meter band and sometimes above and below. I used a Autec RX Vector Analyst Model VA1 (commonly known as an Antenna Analyzer) and an Autec WM1 SWR/Power. I ran tests mostly at the base of the antenna meter, but also some at the operating station. The feed line is 84' of LMR-400 and 16' of RG-8X. I verified results of various lengths of radiator/radials using EZNEC ver 3.0.42. >There were some computer models that suggested that a small number of >radials elevated 1/32 wave or so above the ground had the same effect a >few years ago. Although such installations "worked", when attempts were >made to compare actual field strength measurements of the ground and >elevated systems, the ground systems showed stronger field strengths. > >I can also point to the study that LB Cebik published recently in the NCJ >trying various models of ground radial systems in various modelling >software. The most intriging thing about this report is how widely the >results varied. > >Bottom line: I wouldn't trust the computer modelling of radial systems >within 1/2 wave of the ground as producing 100% accurate results. >Corroboration with actual measurements is needed. I agree. The model does not produce results 100% accurate. It doesn't completely describe the ground system, and doesn't model the interaction between the wire radials and the actual ground completely. However, models are getting better, and produce surprisingly accurate results. I made every effort to verify model results, and usually it was the other way around. I took measurements first then ran the model to compare them. > > 3. When I modelled the > >antenna using EZNEC, going from two to 16 radials did not improve the > >performance at all. In fact, the model showed that 4 radials was less > >efficient than two. If you like, I will send you my EZNEC file. > >It does not surprise me at all that two 2 foot radials is >indistinguishable from 16. Make your radials at least 1/4 wave long, and >you'll see substantially different results. I didn't use 2' radials and didn't model them. My offer to send you the EZNEC files still stands. >And again, the model is just that -- a model. It isn't reality. With the last dimensions given above, model shows a resonant frequency of 14.35 MHZ, an impedance of 38 ohms, and an SWR of 1.25:1. Measured resonant frequency is 14.19 MHz, impedance is 49 ohms and the SWR is 1.02:1. Measured Z at 14.0 MHz is 44 ohms and SWR 1.11:1, at 14.4 MHz Z is 59 ohms and SWR is 1.20:1. My next action is to determine the efficiency of the system then I will run tests with the radials at 6' or 8'. I understand I the system has ground losses, just like a 1/4 vertical with buried radials. That's the principal reason for the differences between the model results in impedance and the actual results. >Bill Coleman, AA4LR, PP-ASEL Mail: aa4lr at arrl.net >Quote: "Not within a thousand years will man ever fly!" 73, Chas, W1CG K2 #46 ++++++++++++++++++