[TowerTalk] Ground Resistance

Wed Aug 9 10:20:27 EDT 2006

Yes, it can be quite easy...not for precision but for "effectiveness".

Forget the megger...other than an intellectual exercise, it will tell you 
nothing about the effectiveness of your radial field for RF.

If you don't have one, borrow an MFJ-259B or 269 antenna analyzer.

With no radials attached, measure the feedpoint impedance at resonance (X=0 
or near 0).
Write it down (it could be 50, 60, 70 ohms or so for a 1/4 wave vertical). 
YOU MUST MAKE THIS MEASUREMENT AT THE FEEDPOINT, NOT IN THE SHACK!)

Now, start adding radials, two at a time.

Put down two (opposite directions, 180 degrees apart), re-measure the 
feedpoint Z at resonance and then keep adding radials two at a time evenly 
spread out on opposite sides and remeasuring feedpoint Z. DON'T FORGET TO 
ADJUST THE ANTENNA ANALYZER FOR X=0 EACH TIME. (The resonant freq will 
likely change slightly each time you add radials, so you have to re-zero for 
resoanace (X=0) each time you take a measurement.After you have done 8 or 
so, start adding them in 4's so you don't have to take as many measurements.

Here is what you are likely to see (something like this, I just picked 80m 
for the example), and assumes approximately 1/4 wave radials (but there is 
no resonance in the radials, so don't worry about precise length)

Freq     # of Radials    Input Z (at resonance)
3720     0                    67
3725     2                    61
3727     4                    53
3728     6                    49
3729     8                    44
3730    16                   41
3730    32                   39
3730    50                   38
3730   120                  37.5

What is this telling you?

1. We already know that the input Z of a 1/4 wave vertical at resonance over 
a PERFECT ground is 37 ohms. This is our "target".

2. Any difference between the 37 ohm ideal and your measured Z at resonance 
is largely ground loss in the radial field (JUST WHAT YOU WERE TRYING TO 
FIND OUT, NO?)

3. Notice that the feedpoint Z drops quickly at the beginning (with very few 
radials) and much more slowly at the end (with many radails). This will show 
you the "point of diminishing returns)

4. Notice at 3727 KHz, with only 4 radials, you have 53 ohms input Z...WOW! 
Flat 1:1 VSWR...ain't it neat! No, it isn't neat at all. The antenna is 
contributing 37 ohms, and your losses in the ground system are contributing 
16 ohms ....rotten efficiency. In the case of a 1/4 wave ground mounted 
vertical, a 1:1 VSWR is a BAD SIGN, not a good sign. Ideally, you should 
have a vswr of 1.4:1, which would indicate 0 ground lossess and 37.5 ohms of 
radiation resistance.

If you want a real thrill, measure the 2:1 VSWR bandwidth each time. You 
will find the widest (best match over frequencies) 2:1 bandwidth in KHz with 
NO RADIALS. The more radials you put in, the NARROWER your 2:1 vswr 
bandwidth will become. Why? Because the earth losses by the inefficient 
radial field are resistiive and not frequency sensitive. Your ground system 
is acting like half a dummy load, swamping out the frequency sensitivity of 
your antenna by putting a resistor on it. As you reduce your ground losses, 
your bandwidth (vswr) narrows. When you bandwidth no longer narrows, you 
have enough radials. (This is similar to: when your input Z at the feedpoint 
stops dropping, you have enough radials).

So, the point of this exercise (I've done it many times, it is quite easy) 
is to find the point at which adding more radials does not produce a 
significant drop in the feedpoint Z at X=0 (with your handy antenna 
analyzer). Depending on your ground characteristics, this could happen with 
as few as 16 or as many as 64 1/4 wave radials (or, many more shorter 
radials).

The seminal work by K3LR on optimizing radial fields is ABSOLUTELY ESSENTIAL 
READING before putting in any buried or on ground radial field. It will save 
you time. It will save you money. It will save your back! What it tells you 
is how to get within 0.5 dB of a 120 radial field. It will tell you how many 
radials it will take and how long they should be. Another thing it will tell 
you is if you have available to you 1500' of wire for radials, how many and 
how long should they be to give me the best efficiency that I can get with 
this amount of wire.

I have very good soil here in Iowa. With my Inverted L, I am currently using 
26 radials 50' long for 80 meters. I am showing efficiency of greater than 
80% (this number is an approximation, not a perfect measurement, as there 
are pitfalls translating Zin to efficiency directly) with this setup. I have 
another 1000' of wire laying around, so I'm going to put in 20 more 50 
footers and that will be sufficient.

I can't emphasize enough reading the K3LR study. There is a VERY simple 
formula for calculating  radial optimization, and it is completely 
consistent with the foundational study by Brown, Lewis and Epstein that has 
been the bedrock of radials studies since the 1930's when it was done. Use 
it. Then, add the radials as I describe above, writing down the Z 
measurements and you will know more about your ground system than 99% of the 
rest of the hams in the world. (And you didn't need a megger)

Warning: there is a program on the net called RADIAL_3, that purports to 
show efficiency of radial systems vs length and number. Do Not Rely on It. 
It does not agree with BL&E, it doesn't agree with K3LR, it does not agree 
with NEC-4 computer models, and it doesn't agree with W8JI's actual 
measurements. It predicts high efficiency with VERY short radials in high 
numbers (like 5 meters long on 80m).

If you want high efficiency in terms of loss, money and effort, follow the 
K3LR guidelines.

If you do what I suggest, please post your measurements on the list. It 
would be most instructive. You need to tell us what kind of antenna you are 
using, of course. If it isn't a plain 1/4 wave vertical, then the "target" Z 
may be different, but the process is identical.

Hope this helps, have fun.

73,

...hasan, N0AN

----- Original Message ----- 
From: "John Cowan" <w0tmm at yahoo.com>
To: <towertalk at contesting.com>
Sent: Tuesday, August 08, 2006 5:08 PM
Subject: [TowerTalk] Ground Resistance

I have followed the posts on radials and ground resistance for some time. Is 
there any simple way to measure the impedance of the ground radial system as 
you install them to know when you have enough of them in the ground?

John

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