[600MRG] A couple of antenna issues...

[Pat Hamel]

Ken,
For 600 meters;

You can build it, put power to it, measure field strengths, and then
calculate ERP.
The measuring method is most accurate, but can be expensive if you hire a
professional company to do the measurement. 
You can build a field-strength meter yourself, study the method of finding
good locations at the correct distances, and take the time and trouble to do
the measurements, and adjust the power until the readings are correct.
-=-=-=-
OR
-=-=-=-
The power to a vertical antenna to achieve the maximum ERP depends on the
electrical height of the antenna which determines it's efficiency which can
be related to the radiation resistance of the antenna.
Figure the electrical height from the ground to the tip or top loading - not
slant length. 
The engineering protocol is that the vertical antenna height is up to the
top loading, if no top loading, then the tip.
Look up the radiation resistance for a lossless antenna of that height (not
the slant height) over perfect ground (I used the graphs in Jasik first
edition). 
Then measure the total input "R" you have.
My antenna (50 foot high) had a calculated radiation resistance of zero
point seven seven ohms.
I measured 28 ohms input "R". The efficiency then was about 3 percent.

Use the writeup by Fritz in www.500kc.com to calculate the correct antenna
current through your perfect radiation resistance and build a transmitter to
push that amount of current through the total measured "R" (including all
the losses).

You will be close to maximum ERP but not above it.
73,
Pat W5THT & WD2XSH/6




-----Original Message-----
From: 600MRG [mailto:600mrg-bounces at w7ekb.com] On Behalf Of Kenneth G.
Gordon
Sent: Thursday, September 10, 2015 12:27 PM
To: 600mrg at w7ekb.com
Subject: [600MRG] A couple of antenna issues...

OK. I have decided to quite fooling around, trying to find some means of 
definitively calculating and working out the exact parameters for an antenna

here, and have decided to do the following:

1) Construct 1/2 of a trap-dipole, exactly like this one:

https://www.arrl.org/files/file/Technology/tis/info/pdf/9207035.pdf,
installed as 
a "sloper" to the top of the 100'+ grand-fir behind our home. fed with 50
ohm 
LMR-400, then

2) Add a trap for 160 meters onto the end, then

3) Add as much wire to take up the rest of the available length 
(approximately 140 feet overall), then

4) Install counterpoises, tuned for the various ham bands, and other ground 
wires, as much as I can lay down, then

5) Add EITHER a top-loading coil, OR attempt remotely switchable 
bottom-loading to enable use on 630 meters. (I would prefer top-loading, but

that may be difficult.)

6) Install a remotely-tunable and switchable bottom-"tweaking" coil.

So, a question or two:

A) How would I calculate the required RF input to this antenna to achieve
our 
licensed ERP?

B) Anyone care to hazard a guess as to this proposed antenna's efficiency 
on both the HF ham bands AND on 630 meters? I would imagine that it 
would "acceptable" at HF, but very poor at 630 meters.

Polarization would be, mainly, vertical in all cases.

Ken W7EKB XSH/24

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