[TowerTalk] Antenna and Radial current

Sat Aug 29 20:08:36 EDT 2020

Dave,

I went thru this with my 160m elevated T, measuring current in each 
radial.  With my MFJ RF current meter calibrated on several ranges I 
found the 125ft 10ft up radial currents varied about 5 to 1.  None at 
zero though, which would be my first investigation also.  The vertical 
is a #13 Davis so easy to measure amps and the radial currents summed to 
very close to the vertical current.  My radials are 12.5ga Al electric 
fence wire. Your radial currents sum to 46.1 vs the 41 you expected, 
maybe close enough.

I have some R65 still on the ground, prior use AM BCB and the bottom 2x 
10ft are welded on full coverage steel sheets, and welded together 
flanges. I thought the sheets were there as anti-climb, but perhaps more 
as conductors.  I was told welding flanges was common BCB practice, I 
would assume mostly on high current sections.

At your low power errors might be significant, IIRC I was using about 
50w, not much of a concern at 1.8MHz.  Your presence might be significant.

In my case radial proximity to a steel building, ending near a tower, 
routed thru numerous trees and a couple in the open is why I think the 
currents are different (highest to the tower, then nearest the 
building).  Exactly how I would equalize the currents is unknown to me.

There is some "expert" wisdom that fairly strongly advises making radial 
currents equal, although I haven't seen analysis to back up those 
assertions. Arguments were, big pattern issues and I^2*R losses.  The 
extra I^2*R losses are easy to calculate and were insignificant. If you 
don't believe in my NEC4.2 modeling then anything can be claimed.

So I took my T model and inserted a source in each radial equal to the 
measured current and checked the pattern, swr, etc.  Maybe 1db skew from 
azimuth symmetry and no significant change elsewhere.  My model with my 
3 other towers present shows more skew.  So, end of my concern until I'm 
educated more about this issue. My path losses due to fairly dense 
forest is much more of an issue, per the recent QST article, but not 
there isn't much I want to do about that.

Grant KZ1W

On 8/29/2020 11:23, Dave Sublette wrote:
> Having followed the Inverted L discussion and learning lots, I decided to
> document my 160 elevated ground plane antenna.  It has been up for thirty
> years.  I replaced the feedline last year. It is 200 feet from the shack
> and I ran a new run of half inch superflex through an underground conduit.
> 
> I have been very happy with the performance of this antenna.  I work the
> CQWW160CW contest QRP, an honest five watts output and place in the top ten
> in North America and top 20 in the world. If I weren't too old to stay
> awake a little longer, I might do better.  Hi
> 
> I hate long detailed emails, but I think you should have an idea of how the
> thing is made before I ask for your advice.  The tower is Rohn 45 with a
> length of four inch irrigation pipe at the top to adjust for best SWR.  SWR
> at the moment is 1.6:1.  The feed point is a Rohn Broadcast antenna section
> with the insulators at 27feet.  Eight Radials are attached to an aluminum
> plate which is insulated from ground also.  The SO239 connector has three
> #12 wires, one to each leg.  The radials are each tied off with an
> insulator and the pigtail fastened to the aluminum plate with threaded, #10
> screws and terminals.  The radials are #13 from Davis RF, insulated.
> 
> So I took my KX3 and a battery pack up to the feed point and took data in
> each wire, while transmitting with 5 watts.   I expected to find the RF
> current to be divided in thirds in the radiating element and by 8 in each
> of the radials.  I had previously calibrated my RF current meter at five
> watts to a dummy load on the bench.
> 
> On my RF Current meter, 5W to a dummy load yields 41 micro amps on the
> meter.
> 
> If the system works as I expected, I should see13.66 microamps in each wire
> to the tower and 5 microamps +  in each radial.
> 
> What I measured was unequal currents in the three wires to the tower, 13.1,
> 9.8, and 15.6 microamps.  Close on the total, but not equal.
> 
> The Radials measured, 9.8, 6.4, 0,1.8, 26.2, 0, 1.9 and 0  microamps.
> 
> My first thought is to go back up and clean up all connections, replace
> crimp terminals on the radial ends and re-measure.
> 
> Additionally, I should be able to take an antenna analyser up (VA-FA5) and
> measure the behavior of each radial.  I expect to see differences because
> of variations in droop angle, nearby objects (barn), etc.  But I don't know
> what to think about the radials that measure 0 and less than 2 ua.
> Does anyone have any further suggestions for how I should go about testing
> and troubleshooting this system?
> 
> I am totally surprised by this result, since this antenna works (but we all
> know that this is an inaccurate and unscientific statement).
> 
> Thanks for your patience, if you made it this far.
> 
> 73,
> 
> Dave, K4TO
> _______________________________________________
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