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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