How to measure & quantify effectiveness of a ground/counterpoise?
Short of trying to take RF field strength measurements, how can I measure the effectiveness of a boat's RF ground or counterpoise in the HF bands?
I would like to try to get some idea of the effectiveness of this: kiss ssb which seems to be gaining some popularity amongst cruising sailors.It appears to be a bundle of "tuned" length wires and coils about 10 feet long which is supposed to provide an easy to install and effective counterpoise for a backstay (monopole vertical) antenna and automatic tuner system in a boat. So far all I've seen is anecdotal "my signal is strong and clear to sailors in east podunk" evidence that is superior to traditional approaches, or even a handful of wires thrown into the bilge.
Any ideas welcome!
If you mean an RF ground in salt water it takes almost nothing to obtain a ground having less than 1 ohm of ground loss. A few square inches of metal in contact with the ocean and you're ready to go. No doubt there will be those who doubt this and so I'll post the math behind it.
Thanks that is illuminating. The old, traditional recommendation - still found in the ICOM marine radio manuals is 100 sq feet of copper in the bilge against the hull. Many people string 4" copper ribbon all about the boat against the hull. At the other extreme, one Gordon West demonstrated with a light bulb in series with the antenna that attaching a ground wire to a bronze through-hull resulted in more current to the antenna than the 100 sq ft of copper. Now the vendor above is marketing his $145 solution as equal to or better than most other traditonal solutions, most of which would be much cheaper, some just as easy. I'm just looking for some answers!
I've worked East Podunk several times with very strong signals.
I agree empirical trials and data are great, and field strength readings are the way to obtain the data.
I'd want to set up an FS system on shore and be at sea, and have either remote telemetry or an assistant on shore to read back the readings to me as I make changes. Problem is you really need a tunable and calibrated FS receiver to make any sense of it.
Still, this would be very enlightening.
I've never been a sailor but have operated "at sea" a few times and my impression is that it doesn't take much on the open seas to work the world. Operating from the upper deck of a Carnival Cruise lines ship going down the Pacific coast of Mexico a few years ago using a totally crappy "Diamond" (Japanese) multiband HF mobile antenna on a C-clamp that was clamped to a brass railing around the inner perimeter of the deck wall and 50W PEP from my Ten Tec Scout (battery powered), I was working into Europe with great signals and every contact reported "great signal," whatever that means.
My impression as a non-sailor is that "the open sea is a great place to use radios."
While salt water makes for a great RF ground, it's not so easy to get good contact with that salt water from a fiberglass sailboat that is for sure. I played around a bit in the 25+ yrs I was a live aboard. I finally wound up with one of those big "expensive" slitered bronze ground plates - seems like it was 8" X 18" or something? Real big anyway - ran flat copper strips about 2 or 3" wide from autotuner under backstay to ground. Then from autotuner I had a #8 AWG wire ran under the caprail up to bow, then around to other side to give about 60 ft of wire. The back stay was about 50 ft of insulated portion (is is 40 ft of backstay and about 10 ft of #14 wire from bottom insulator on backstay down to autotuner.
That would work as well as anything I came up with. I looked over the page you linked to - sure sounds like snake oil to me.
Antennas and RF ground systems are most all compromises on boats! The best used was a 40 meter dipole in inverted Vee fashion, but that was hard to sail with - only for anchorages.
Let us know how it works. If you'd like to set up a schedule to test on I'll be happy to help - I'm in Mobile, AL and have a couple of antennas to listen with, and a 6 band hexbeam. Yep, I finally swallowed the anchor.
fair winds and quartering seas,
I have a suggested method for testing ground effectiveness based on AM broadcast antenna ground systems. The feedpoint resistance of a vertical radiator over ground will decrease as the ground is improved. Specifically, as the number of ground radials are increased the resistance drops. At some number, the resistance does not get any lower or significantly lower. The FCC requires 120 radials but tests by hams (see QEX and QST) have shown that above some number, not nearly 120, the resistance is for practical purposes as low it will get. Why can't the same measurement technique be used for HF boat antennas? I can't see any reason why it would not work and it would be easier to do than FS measurements. An MFJ 259 is an inexpensive instrument that should be able to handle the job. The idea is to measure the resistive part of the impedance. I think a 33 foot piece of wire for 40 meters fed against the ground scheme to be tested would suffice. I am not a boat owner and never was on board anything other than my kayak, I think that is short for a typcial mast for a sailboat If the ground is very good, the resistance will be down about 35 ohms which is about the best you can expect for a 1/4 wave vertical over a ground plane. Remember to take into account the electrical length of any transmission line between the test instrument and the feed point. A Smith chart of TLW (transmission lines for Windows) will work for taking out the transmission line. 73
It's been many years since I was involved in this, but AM BC stations used to be required to take actual ground-level FS measurements and report them back to the FCC periodically.
Originally Posted by WB2UAQ
That not only confirmed the effectiveness of the counterpoise systems, but also confirmed things haven't changed with local construction of buildings, bridges, other towers and so forth -- and those things always changed.
Used to be, FCC was only concerned with ground wave coverage for AM BC, and sky wave wasn't really a consideration since that was incidental and unintended.
I like that idea of measuring the resistive component of the antenna and comparing. It seems though that resistive losses withing the counterpoise itself - which could be higher in the KISS counterpoise due to its construction - might skew those results?
I'm thinking that I may end up trying some kind of RF field comparison too, but that is fraught with difficulty with the boat because changes in the nature of the ground/counterpoise may significantly change the direction of radiation lobes even if total power output were the same.
The losses would be part of the resistive portion of the impedance. There's no way to separate them out. Good point about the changes in the FS due to the fact that you are changing the radiation pattern when you change the counterpoise.
If the installation is a sail boat antenna input impedance is a difficult (but the simplest) way to measure the ground resistance. All of the rigging gets into the act and alters the monopole input impedance. To get around this a 130' wire can be flown with a 36" helium balloon and tested at 1.8 MHz. The rigging will not affect this too much (I think). Best to run a NEC model first.
I operate from a sailboat every ARRL Field Day and have modeled the boat and several potential antennas. A monopole excites RF current on all of the rigging making for very odd radiation patterns. I also operate from the Great Salt Lake of Utah running a 130' balloon antenna on 1.8 MHz. The ground resistance at the lake is about 1 ohm with the connection to the water being a 6" length of 1" copper pipe.