This has been a good discussion, I am currently building a 40 meter vertical
with raised radials. The radials are in the "working" position now @ 4' and
will be raised to 8' at the perimeter and the feed point will be 12' giving a
downward slope. Jim, I have also studied Rudy's page for a couple of
months...there is more information in there than the casual reader would notice.
My only drawback is that the vertical is being constructed in the woods.....
Work has been postponed until Erma has passed....what a waste...nice weather
today in Georgia....
Ken
N4zed
-----Original Message-----
From: TowerTalk [mailto:towertalk-bounces@contesting.com] On Behalf Of Jim Brown
Sent: Friday, September 08, 2017 8:20 PM
To: towertalk reflector <towertalk@contesting.com>
Subject: Re: [TowerTalk] Tuning raised radial verticals
On 9/8/2017 3:19 PM, Grant Saviers wrote:
> I was motivated by Jim's post to dig deeper into measuring my 160m T
> antenna performance. A number of questions herein for experts willing
> to dig thru this.
>
> My 160m T has 8 elevated radials and I measured the currents, they
> vary about 3:1. That seems significant. Given the "make radial
> currents equal" advice, what performance am I losing with unequal
> currents? How do I tweak my EZNEC Pro4 model to generate unequal
> radial currents without introducing loss? Then, if there is more than
> a db or 2 azimuth asymmetry from the 8x 120' radials how would I
> equalize the current in them?
All of these questions are answered in N6LF's 2-part piece in QEX. The short
answer is that power lost is I squared R, and if current is much higher on one
radial than another, the loss increases as the square of the current.
>
> The T is PE insulated Davis CCS 13.5ga. All 8 radials are elevated
> 10' using #12.5 aluminum electric fence wire and are 120'+/- 4' long.
> Pretty much a sweet spot in the N6LF analysis.
As I posted here several times, including in this thread, N6BT advised me that
16-20 ft is a minimum height for 160M, and someone else posted a guideline that
suggested even higher.
> The initial as measured currents sum of radial vs vertical was
> different by 0.13a. This led to a calibration of the MFJ which
> discovered the 1a scale used for the vertical current measurement was
> reading high by 20%. The 100ma and 300ma scales were within 5% except
> below 30% of full scale, which I think is error from the sense diode
> forward drop.
A suitable RF ammeter is easy to build -- all it takes is a coil around a
ferrite clamp (like the MFJ) driving a DC microammeter through a rectifier with
filter capacitor and current limiting resistor (which can be a pot).
>
> I agree that for 2 to 4 radials equal currents are desirable, unless
> an azimuth gain skew is desired as with a CrankIR "on the beach".
While skewing of the horizontal pattern happens, the primary concern is loss in
the radials.
> Somewhat related is AC6LA's sample modeling with 8 equal radials shows
> minimal differences in azimuth gain (0.1db) due to the coupling to the
> T top wire for a T similar to mine.
> https://ac6la.com/aecollection3.html
>
> It also seems to me that there is a number of radials at which the
> concept of radial resonance makes little sense. It is like asking
> "what is the radial resonance of a large sheet of perfect conductor or
> for seawater under a vertical". And then as is seems likely in my
> case the metal structure couplings (towers, building) are the reason
> the nearby radial current is high, how would changing the resonant
> frequency affect the current? Or would it be better (rhetorical
> question) to remove high current radials to force more "equality" (the
> radials are pretty much equal angular spacing as installed)? Or add
> more radials where the currents are low?
>
> Lot's of questions when a model meets a measured real world situation!
> Am I missing something?
Yep. Study Rudy's piece again. :) I've been through it a couple of times, and
spent about 4 hours each time.
73, Jim
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