[TowerTalk] Tuning raised radial verticals

[Jim Brown]

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