A few years ago we had a horrible hurricane season. The first one through was Charlie, and he broke, one of my 100 year old oaks with a trunk 6 feet in diameter like a twig. It was a shame, that tree added at least 10,000 to the value of my property and cost me a grand to have removed, BUT it opened up some space in the back so instead of acorns I planted aluminum.
I started with Butternut HF-2V I had laying in the garage. Of course with verticals comes the constant conundrum, HOW MANY RADIALS. If you read the EHAM Elmers forum very much, that question is asked about twice a week. Radials are like prunes. Is 6 enough is 12 too many??? It all depends, AND like prunes, you need at least as many as you need, but you really don't need any more. You can read ON4UN 's book or a million other sources or try and gleen the goat entrails to answer this question. The argument on EHAM usually goes: "I got ten and I worked England!!!" Then some joker whips out the ARRL hand book and whacks you with it...120....120....120... 120 times. 120 probably comes from the FCC's proof of performance for AM broadcast stations. If you string 120 of the suckers, they won't make you do a full proof of performance, which is a real PITA. Hams are not responsible for proof. Hell we're just amateurs. Like girls, when the working day is done, we just wanna have fun.
The real gold standard of how a vertical performs is to measure its field strength. It really has nothing much to do with impedance bridges or SWR's. Those things may be strongly or weakly correlated with performance, but what wiggles the other guy's S-meter is field strength. I planted about 12 radials as the Butternut manual suggests and promptly worked England on 80. I had a lot of fun that winter making contacts. I worked DXCC on 80 in about 3 weeks. But I was always bugged by the prunes question. Over the course of that winter I aquired my SDR-1000.
The SDR-1000 has a wonderful receiver. The AGC is dead on accurate with no over shoot, and the RX is absolutely"linear" (if you can call something logarithmic "linear") in the way it reads. It reads out in dBm from MDS (-135dBm) till the RX runs out of head room somewhere around +2 dBm (73db over S9), one measly dBm at at time. This is a true 73 db over S9. Most radios would have folded long ago. Signals that would run 40 over on most radios (because of their poor linearity) usually only run S9 to 10 over on this radio, meaning there is a whole lot of head room up there. It also runs off 12V as in car battery, and the "sound card" can run off the fire wire port without an external power supply. So I took a laptop and the SDR-1000 and hooked it up to my car and parked my car at the front corner of my property and flipped a little wire up in a tree. The laptop was connected through my LAN and I had another laptop at the base of my vertical also connected to my LAN. With TightVNC, I was able to control the rig in the car from the base of the antenna and I could record dBm as I fiddled with the antenna. I put a signal gen on the antenna and tuned the RX to that freq.
I had a ground rod at the base of the vertical and started my measurement there. As I added radials I noticed that when I connected the ground rod to the radials my field strength went DOWN. What the heck was going on???? I expected my strength to go up. I added wire and added wire and eventually around 25 wires when I connected the ground rod there was no difference in field strength. I added another 15 radials just to see if there was any difference. At 25 radials my base impedance also stabilized and the addition of more radials really didn't change the base impedance or SWR bandwidth. I say this as a conformation test and not as the major thing that was being measured which was field strength.
Here is what I discovered. A vertical pipe when excited with RF induces fields into the ground and it is the point of this radial field to recover these currents and return them to the other side of the coax. What you want is a really low common mode impedance between ground and the coax. It is my impression the radial field acts as the top plate of a capacitor, and the earth acts as the dielectric and bottom plate. As you add radials you effectively couple into the ground. Once you are completely coupled into the ground no more coupling makes any difference. I think the reason I saw a decrease of field strength when I attached the ground rod is because that made a current divider when part of the current in the radial field was shunted back into the ground, and part went to the coax shield. As the radials increased eventually there was no longer a potential difference between the rod and the radial field. Both were effectively at "ground" potential, and all the current was arriving at the shield of the coax. ("All" of course is a relative term, but in terms of dBm any additional return was asymptotic) I had my low impedance common mode path between the coax and the currents that the monopole was inducing into the ground.
As a first iteration before I went the SDR route, I tried using my Elecraft K-1 as a detector. Unsatisfactory. ACG and S meter were not accurate enough, and too much running around between the car and the antenna. The lab quality RX in the SDR and networking ability made collecting this data possible since I could visualize what was going on quite easily. Later I put up a full size vertical on 80, and I tested again. This time I parked my car on my buddy's yard about 3 blocks away. I put up a 10ft probe antenna hanging off a branch of a tree using the car as the counterpoise, and I used a WIFI tin can wave guide antenna pointed to my QTH. At my QTH I had a WIFI repeater on a tripod connected to another waveguide antenna up on the roof of my garage. I wanted to see if there was much difference being close to 10 wavelengths away instead of 2 using the big antenna, plus I wanted to screw around with long distance wifi. The radial field was about the same under the big antenna. At 25 radials there wasn't much change and I really couldn't tell if there was a difference between 2 and 10 wavelengths because the 66ft pipe is a better antenna, and I didn't have the HF-2V up anymore. The bandwidth of the big vertical was dramatically larger than the short and my impression is that the antenna is more efficient. Maybe one day I will give that experiment a try between the car parked in the front yard and over at my friends house. The new F1.5 K should be perfect for that test.
I'm not suggesting 25 is enough. I am suggesting 25 is enough for me given my soil conditions. I use this radial field for multiple verticals, the 80M 1/4 wave, a 40M end fed half wave, and a 20M 5/8 wave, plus an inv-L on 160. I use the 80M antenna on 30. I use a relay box to switch antennas and find virtually no interaction between the 4 antennas. I later added some 120ft radials after I added 160, just because I could, but I didn't take the time to do any 160 measuring. 4 verticals, 5 bands, one pipe, and one radial field I love it when a plan comes together.
So here is an example of how the SDR-1000 was able to be used as a remote lab quality receiver helped me answer the age old conundrum, and gave me the chance to spend some time out in the yard having some very interesting spring time radio fun. What's the difference between 25 radials and 120? In my case a lot more money and a hell of a lot of work and really no more signal wiggling the guys S meter on the other side of the world. I've put in several 120 radial fields before and its a project.
One afternoon as I was running these tests I walked into the shack about 12:45 local and did some tuning up using the amp on the low end of 80. Low and behold I was called by VK6HD at 1 friggin o'clock on a hot Florida spring afternoon. I was floored. He was perfectly Q5 and the only signal on the band. He confirmed. What a hoot!!!
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