[600MRG] Antenna efficiency question

[Pat Hamel]

Jim,
The efficiency of your inverted L antenna is determined by the radiation
resistance of the vertical portion. The horizontal portion is considered
only top loading at this frequency.
To determine the radiation resistance (NOT the measured total resistance)
you can use the published information by figuring the electrical length of
the antenna for an antenna over perfect ground with no trees to soak up your
RF.

Without digging out the logbooks tonight, my 50 foot antenna had a
calculated radiation resistance of zero point seven seven ohms. The input
resistance (measured where the antenna left the tuning box) was in the range
of 38 ohms R and X=8.
The power that the antenna actually radiated would have been 0.020206 or two
point zero two percent. 
My four push-pull-parallel 811s never had more than 1200 volts available, so
I had about a KW DC input to get pure linear class B 300 watts out to get
around 15 watts ERP. Remember - we were trying to prove that we could
operate without QRM to broadcast . The second and third harmonics are in the
AM broadcast band and old people still listen to the "Milking Music"
stations - some of then can copy CW.

To calculate your radiation resistance, go to www.500kc.com and then the
technical downloads. There are good files there. The reading and math should
take less than one evening.

HOWEVER - top loading makes a lot of difference. I used 11 twisted pair #20
wires 120 feet long spaced on 10 foot copper pipe separators above the
loading coil which hung from one spreader.
Before I had to make a decision to stop 600 meters I was planning to
increase the radiation resistance by changing the vertical radiator from 11
parallel wires in a one inch bundle to a six-inch cage which would have been
more efficient. Hanging the coil at the top of the vertical will act like an
RF choke and let you use the antenna on 80 meters. I am attaching pictures
which won't go through the reflector to the group.

Look out for the end insulator. The cycle at 600 meters is long enough to
arc a HF four-inch insulator on a damp day. I used foot-longs. There is a
file of pictures on W5JGV.com with burning insulators you should see.

Enjoy the feeling of accomplishment & 73,
Pat W5THT & WD2XSH/6
P.S. the loading coil and amplifier are very affordable to someone who will
USE them.
Many of the parts were donated, so I am not looking for a profit.





-----Original Message-----
From: 600MRG [mailto:600mrg-bounces at w7ekb.com] On Behalf Of Jim Miller
Sent: Wednesday, April 20, 2016 6:27 PM
To: 600mrg at w7ekb.com group
Subject: [600MRG] Antenna efficiency question

Is there any "rule of thumb" that would give me a ball park number for the
expected efficiency of my 67x67ft inverted L that I now use on 160 if I
just added a loading coil at the bottom?

If it was (or could be) 10% and the FCC only grants 1w ERP when they give
us an official band then a 10W amp would be all that is needed.

OTOH, if it is likely to be closer to 1% then 100w would be needed.

TIA

73

jim ab3cv
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