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WB4C
06-23-2008, 05:49 PM
I've been experimenting with a 3 foot elevated above ground short vertical with a small chickenwire groundplane mostly on 20 meters. I was wondering if the difference in relative field strength, from the antenna, between a galvanized steel groundplane and a similar structure of copper would be measureable and/or significant?

I removed the chickenwire screen (.05 wavelength) and ran with six 7 foot radials(always there) in an "x" pattern with no degradation in signal strength. (I'm not surprised) A .1 wavelength screen was very iffy, with and without radials.

I am currently playing with putting various sizes of aluminum foil underneath the insulated radials; attached and not attached; fat, skinny, short and long, at the end or at the base. Very interesting stuff and up to a + or - 1.7 db change, depending on how done.


For right now I better find out if the galvanized steel was holding me back? Too lossy?


The antenna is 7 feet tall, has a large top hat and no loading coil. It's matched at the base with a MFJ tuner. The "older version" (smaller hat and mid loading coil) beat the 20 meter Hamstick by 3 db, if anyone is interested.

73, Jim WB4C.

W0EA
06-23-2008, 06:04 PM
Sounds really REALLY crappy either way. 7 feet is less than 1/8 wave on 20m... you're probably putting almost all your power in the tuner! You didnt mention how long the vertical part of this antenna is. My guess is, you could do a million times better with a simple dipole instead.

WB9JTK
06-23-2008, 06:50 PM
The theorists will tell you (correctly) that you are better off with copper than aluminum than galvanized steel. (zinc coating on the steel (galvanized) is a crummy conductor and even worse at RF).

The calculations and measurements that you can read about in the ARRL Handbook (and other places) show that if your radials are not long, you don't need many. Huh? In other words, if you cannot make them long, there is no point in putting down a lot of them. So a few short radial wires will work just as well as a perfect ground 'screen' when the radius is quite short.

In your case the most important thing is corrosion. You don't want to have to replace your 'ground' every year. Not just plain old corrosion, but corrosion due to dissimilar metals connected in a wet environment. If your antenna base / mount / whatever is aluminum, then aluminum wire would be your best choice. I used to put down copper wire and connect it to the aluminum with stainless steel clamps etc (stainless steel between copper and aluminum).

However 4 years ago I put down galvanized steel 'chicken wire' ground in my yard. It is already rusted away in many areas. But filling my yard with 'chicken wire' was very fast and easy because of the type of grass we have here. I will have to replace it all soon. In my case it was much easier than putting down a "bunch of radials". Probably in 15 or 20 years I will have to replace the 'chicken wire' a few times so in the long run regular 'radials' would be less work.

Here in Florida we have a grass called "St Augustine". It grows on TOP of the ground. So I just mowed the lawn very very short and laid out the 'chicken wire fence' down on the ground, soldered it all together at the base of the antenna (not to the antenna) and held the wire down with 'staples' made out of #14 solid. In two months the 'chicken wire' disappeared under the roots of the grass.

east & west my 'radials' are only about 10 feet long, north and south they are more than 25'. (I tried to fill a 75' x 25' area) With 100 watt on 80 and 40 I make contacts without a lot of trouble and don't fill my day worrying about what is most effective.

If I ever sell this place and move, I cannot imagine what a non-ham will think when they go dig in the yard and find the conduit with coax and a yard full of 'chicken wire'.

Also, you can compare your home installation to a HF mobile.... how much 'ground plane' do you get out of a modern car ? Not much at all.... but we still make contacts that way.

W8JI
06-23-2008, 06:51 PM
I've been experimenting with a 3 foot elevated above ground short vertical with a small chickenwire groundplane mostly on 20 meters. I was wondering if the difference in relative field strength, from the antenna, between a galvanized steel groundplane and a similar structure of copper would be measureable and/or significant?

The antenna is 7 feet tall, has a large top hat and no loading coil. It's matched at the base with a MFJ tuner. The "older version" (smaller hat and mid loading coil) beat the 20 meter Hamstick by 3 db, if anyone is interested.

73, Jim WB4C.

Jim, I really doubt the material the screen is made from means anything at all except for weathering.

That is a pretty small ground plane though, so I expect you have some errors caused by common mode on the feeder unless you somehow decoupled it.

73 Tom

KA0GKT
06-23-2008, 07:15 PM
I won't ask you what you used to calculate the 3dB difference between your antenna and a hamstick...

A number of years ago, there was an article on QST covering a short, in terms of wavelength, antenna for 40-meters which used a really large capacitance hat plus an inductive load at the center of the antenna. The antenna used 120 1/4-wave radials on top of the ground, IIRC. (It used to be in the antenna book too.)

The biggest problem with short antennas is the radiation resistance is quite low and therefore all of the series resistance must be kept to a minimum in order to get any reasonable amount of power to radiate. Certainly, a top hat is a good thing to do with a short antenna, however the current distribution of a base loaded antenna is going to be poor from the get-go, and the old MFJ tuner will add too much series resistance to the equation to get decent radiation. I am not saying that you should scrap the idea of using the tuner at the base, I'm suggesting that a High "Q" loading coil of sufficient size be added a few inches below the capacity hat in order to get the radiation resistance of the antenna up out of the low single digits to perhaps the high single digits.

Since a short-in-terms-of-wavelength antenna is a compromise at the outset, it would behoove the amateur to use the best radial system possible. If the radials are going to be in the ground or on-top of the ground, copper is the best bet. Sure, the guy with a full size vertical might be able to get by with galvanized chicken-wire, but when you are starting off with the handicap of a physically short antenna, you need to not scrimp on the ground system, IM (not so) HO.

Good luck with your antenna.

WB4C
06-23-2008, 07:59 PM
Thanks all!

You bet there was common mode! A 1:1 isolation balun is a must with .05 wavelength! Rf in the headphones Ha!

Tom: as you pointed out previously, a .05 screen is pretty much of a non-performer. Running onger radials over it is fine and sets up a much needed electric field. If the radials are at least twice the radius of the screen, the screen can be taken out. [at least for a short and slightly elevated vert.]

As long as I wasn't looking at huge losses in the steel, I'd say that the chickenwire groundplane at the base of the antenna was no good. The real action seems to be close to the end of at least a .1 wavelength radial. As I think you said before, a very small but dense groundplane at the base of the antenna misses the point and won't do. Looks like .1 radius is the minimum stable distance.

I'm doing things to the radials I've never seen in any books. Very interesting stuff. I wonder if a screen for this thing should look like a "o" ring or washer, with radials run over it and perhaps terminated at the ends to the screen? That seems to be where I'm headed.

73, Jim WB4C.

K9STH
06-23-2008, 09:11 PM
I have used galvanized electric fence wire for radials for decades. Would copper be better? Yes. In a practical sense can you tell the difference? No. Then looking at the difference in cost between galvanized fence wire and copper wire these days the use of galvanized wire for radials comes out very favorably. Now when buried the galvanized wires will usually not last as long as copper. However, they will last a pretty good length of time.

Before my swimming pool was installed (26 years ago) I had 48 buried radials under my HyTower. After the pool was installed I went to elevated radials (which run along the house, along my wood stockade fence, and along the retaining wall to the pool) and those work MUCH better than the buried radials. Occasionally I have dug up one of the old radials (that were cut off when the hole for the pool was dug) and the wire is still in fairly good condition. Now copper would probably be in better condition, but the radials have been in the ground for almost 36 years.

The HyGain HyTower is an excellent vertically polarized antenna (basically full size on 80 through 10 meters) and the basic component of that antenna is galvanized tower sections. I also have a 40 meter vertical built similar to the HyTower using Rohn/Spaulding tower sections. It works very well. Also, the vast majority of AM broadcast stations use galvanized towers for their antennas and those work quite well.

Now you definitely do NOT want to use aluminum wire for in-ground radials. In most soils within a time frame of 6 months to a year it WILL disappear. I have a friend who, years ago, put 120 full sized radials under his HyTower using aluminum wire. At first the antenna worked great. Then, over time, the performance started dropping off. One day he was mowing his lawn and happened to "trip over" one of the radials and found that it was basically right at the surface of the ground. He then started pulling on the rest of the radials and found that all of them had "rotted away". None of them were any longer than a foot. That was about a year after he had installed the radials. You talk about disappointed! He had spent a "fair" amount of money and quite a number of hours installing the radials.

Glen, K9STH

WB4C
06-23-2008, 09:56 PM
Glenn:
My outside vertical has radials of 21 ga enameled copper magnet wire. they were ground surface mounted and are now undetectable. The ones near the base that I can see are in pristine condition!

I guess the moral is don't worry about the RF resistance of galvanized steel, at least in this application.

Steve:
Thankyou for not asking! Ha!
I've tried two different tuners and no tuner. Not bad, really not bad at all. The only problem I've seen is if the choke balun wasn't dead smack hooked to the tuner, which also needed to be immediately at the feedpoint, the supposed match was boned up and the tuner and the coax pigtail got warm and the RF went down. (smaller the groundplane the worse it got).This would happen less than 2 feet from the tuner(at the feedpoint). I would read a very low impedance(like you would expect) and loose about 2 Db in the process! My thought is that the "T" match can be as lossy as the "Antenna Book" says it might at low impedances. After all that I went thru with this, I wonder how many guys who can't run up to the feedpoint of an antenna quickly enough to feel any heat, have engineered a 2 Db loss into an antenna system that can't afford it. I'm glad I've done all this just so I can see and feel how all this stuff plays out. Anybody can read a book; anybody can pass a test.

73, Jim WB4C

WB9JTK
06-24-2008, 06:14 PM
Now you definitely do NOT want to use aluminum wire for in-ground radials

Depending where you live this is very good true. Aluminum is fine in acidic soil, but in basic soil it will disappear amazingly fast.

In WW 1, observation ballons were filled with hydrogen gas generated where needed. A horse-drawn wagon full of water was pulled to the location, buckets of lye tossed in the water, then they tossed in blocks of aluminum. Huge amounts of hyrdogen were produced as the NaOH broke down the aluminum.

M0DSZ
06-24-2008, 08:07 PM
I have used galvanized electric fence wire for radials for decades. Would copper be better? Yes. In a practical sense can you tell the difference? No. Then looking at the difference in cost between galvanized fence wire and copper wire these days the use of galvanized wire for radials comes out very favorably. Now when buried the galvanized wires will usually not last as long as copper. However, they will last a pretty good length of time.
Glen, K9STH

Can you get green PVC coated, galvanised steel wire? It's popular here because of it's (dis)appearance but also avoids electrolytic corrosion when in contact with other metals or acid soil. Once again I add that my counterpoise/ground system is mostly galvanised stock fencing round a field, mostly on wooden posts. It's long but of no specific length, I'm sure you don't have to be too academic about the length as long as there's enough of it.