[TowerTalk] RADIAL Questions and Answers

[CQ K8DO]

OK...  I blew it...
My post immediately preceeding this was not ready for posting, but was a work
in progress in my edit file... I already knew that I had errors about
impedence changes in one paragraph, especially for bare radials over low
conductivity dirt... I planned to run a test on Saturday and then rewrite that
paragraph before posting...Due to a vapor lock of the frontal lobes this
morning (Monday) I dumped the text to the email screen for further editing,
got into a phone conversation and idly clicked on the flashmail command and
poof -> gone... Here is the offending paragraph...

<snip>
Ground proximity drastically lowers the impedence to the point that
standard bridges are unuseable for setting the resonance point... I suggest
that you configure a pair of the radials as a dipole - elevate them about 3
feet (which will bring the impedence up to where you can measure) and use your
RF-1 or noise bridge to adjust the length to resonance in the middle of the
band, then cut all the radials to that length... Putting them back on the
ground will pull the resonant frequency down another few percent, just about
where you want it...
<un snip>

So lets discuss the experimental data and then revisit that paragraph...

Saturday I grabbed a random length of insulated #12 (solid) wire off the pile,
which turned out to be roughly 46' long... made it into a dipole, took it out
in the driveway, hammered in two tee posts about 46 foot apart and strung this
dipole up between the posts... The height turned out to be about 4' on ends
and about 3' at the center...
I used the RF-1 to find the resonant frequency and the Z (the L and C are
unneeded for this discussion)... Then I dropped the dipole on the ground,
which was wet gravel about 8 inches thick over a sand and clay base, and
repeated the measurements...

Insulated wire dipole
@ 4' high   -->    9.655mc at 54 ohms
On ground  -->    6.741mc at 88 ohms    
The antenna handbook has a set of curves which show impedence changes with
proximity to ground...  

Then I grabbed some bare #14 stranded wire and re-used the insulators from the
first dipole.. The length was (again) approximately 46'... (the tee posts were
already set)

@4'  high  -->     10.030 mc at 53 ohms
On ground -->      6.851 mc at 96 ohms   -  wet gravel  - 
this was a surprise... I expected the wet gravel to have more effect than this
on the resonant frequency and the Z... but, sand and stones are not
particularily conductive, even when wet...

Then I dropped shovels of wet gravel on top of the dipole wire and
remeasured... (total of 8 piles evenly spaced)
Partly buried  -->     5.987 mc at 114 ohms  --  rising Z ???? ....

This was getting interesting so I dragged the dipole over to the new section
of lawn with 8" of exposed wet, black dirt over a clay base, and sort of
stomped the wire into the dirt with my feet... The length was not pulled out
as tightly as out on the drive...
On dirt     -->  6.513 mc at 168 ohms ....  <mutter, mumble>

Hmmm.... OK, so I dragged it over to the pond and sort of laid it along the
shallows, partly submerged and the middle on some boards to keep the RF-1
dry... - The resonance was broad, more than 200kc without any change in Z....
Partly in water --> 2.100 mc (+ -) at 145 ohms...   
This was more like I expected, except for the Z....

CONCLUSIONS:
1. On the ground, bare radial wires can be tuned, BUT, expect changing
moisture levels, and ground freezing, to have drastic day to day effects on
resonance and impedence... And, standing water will totally detune your
system...
2. Short radials over moist soil will be electrically longer than expected
with a velocity factor of 60% (very, very, roughly)...
3. Dropping the radials from 3-4 feet,onto the ground, obviously pulls their
resonant frequency more than a few percent.... (Moxon already pointed that
out)
4.  In salt water very short radials will suffice ( an extrapolation, and not
tested by me)

Discussion:
The RF-1 puts milliwatts into the radials... Will these measurements hold at
higher RF voltages for bare radials on ground?  I have some strong doubts....

This short series of measurements were done on one band... They need to be
repeated for each band from 160 to 10 meters and a series of curves
constructed...

These measurements were made at one location, at freezing temperatures (about
32 degrees) with some frost in the ground), with one instrument (I did not get
to do a noise bridge backup before it became dark)

I am planning a simple series of measurements with a single vertical and a
pair of insulated radials from ground contact to some height above ground at
one foot intervals... More later...

Denny



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