[600MRG] Applying Smith's advice

[Rudy Severns]

I took Smith as gospel!

 

When I built my antenna the first thing I went for was maximum height, I
ended up with roughly 90'.  The next step was to add capacitive top-loading
wire until my tuning inductor was down below 50uH.  At that point the
inductor loss was small, no point in hanging up more wire.  After this I got
very aggressive and laid out 250 150' radials on the ground surface.  My
efficiency seemed to be pretty good.  My antenna is much larger than most
but I applied same principles, in the same order as suggested by Smith which
should serve as an example.

 

One additional note.  My ground system is obviously ridiculous, very few
hams will want to emulate it and many cannot install any significant wire
on/in the ground.  So I asked myself, suppose I used instead a simple wire
counterpoise which would be easy to implement.  How would that affect my
efficiency?  The counterpoise I settled on was simply a four wire mirror
image of the four top-loading wires, i.e. counterpoise wires directly under
top-hat wires.  Going from a ground based system to an elevated one, where
the counterpoise wires need to be at least 8' above ground, I had to shorten
the antenna by10%, which decreased Rr by 20% and increased the loading
inductance to 67uH, roughly double, so I expected to take a hit on
efficiency.  However, careful modeling indicated that I might actually
improve efficiency, which I could not believe but I went forward anyway.  

 

My new antenna has indeed proven to be much more efficient, I've had to
substantially reduce my transmit power to stay with the license limits.
When Larry, XGP, sent me a +27dB WSPR report, down went the power knob!  One
tell-tale sign is that the match bandwidth is much smaller, I have to retune
to cover the entire band whereas before I did not.  The price of efficiency
is higher Q and smaller bandwidth.  The old-timers appreciated
counterpoises, perhaps we should think more about them.

 

The main disadvantage of counterpoises is the requirement for electrical
isolation.  This is a real problem.  My solution was to use an isolation
transformer but I'm not impressed with that and there will be changes, for
there are many possibilities from the past.  If you want the details and
photos of my present antenna go to: www.antennasbyn6lf.com and click on the
new 630m antenna button.

 

73, Rudy N6LF