[Elecraft] Antenna question

[Edward R Cole]

George,

You do not say what power you are transmitting and it would be 
helpful to know the separation between the antennas. Will the 
vertical be as high as the yagi?  Need to know what frequency is used.

OK, in lieu of this data I will make some assumptions and show you 
the math to calculate the power received.
F = 3.5-MHz
Po = 100w = 50 dBm
separation = 60 feet = 18.3m
cross-pol loss = 20 dB (it will not be quite this much at this close 
separation)
L = 32.4 + 20 Log F + 20 Log d, where F= MHz and d= km
L = the space loss between antenna, in dB
L = 32.4 +20 Log (3.5) + 20 Log (18.3/1000)
L = 32.4 + 10.9 - 5
L = 38 dB
Ltot = L + Lcross-pol = 58 dB
Po - Ltot = -8 dBm

That is a very strong signal but will not hurt the 
receiver.  Typically anything under 0 dBm is safe.  I run 1500w and 
have a preamp attached to the TR coax relay and have had no problems 
blowing up the very sensitive transistor in the preamp if the relay 
isolation is equal to the power in dBm.

Now cross-pol loss is probably not 20 dB but more like 15 dB and your 
antennas are in near-field so these formula are not quite 
accurate.  Ideally you would measure the power on the receiving 
antenna with a milliwatt power meter but I suppose you do not have 
one.  Also I did not include antenna gains in this, but in close 
proximity they are probably not accurate.

I have tested my tower mounted preamp by sending a test signal to an 
antenna about 130-foot away and using the space loss formula pretty 
accurately predicted received signal level.  This let me test the 
preamp sensitivity without removing it from the tower.

If you have a scope look at the received RF signal and measure the 
peak voltage.
P = E^2*R or
E = sq-rt (P/50)
P = 0 dBm = 0.001w
sq-rt(0.001/50) = .004v

73, Ed - KL7UW
PS: If you run more than 100w then either short the receiver antenna 
line or use a coax relay to disconnect it when transmitting.

------------------------
Date: Tue, 11 Feb 2014 23:36:09 +0000
From: George Thornton <gthornton at thorntonmostullaw.com>
To: "'elecraft @ mailman . qth . net'" <elecraft at mailman.qth.net>
Subject: Re: [Elecraft] Antenna question
Message-ID:
         <040f2ec01a53458babf11914164d5177 at Server.thorntonmostullaw.com>
Content-Type: text/plain; charset="us-ascii"

This might be a stupid question, but here goes.

I have a small lot.  I currently am using a 3 element Yagi that 
barely fits on the property.  I was thinking about getting a vertical 
as a second HF antenna.  If I put it up it is going to have to be 
pretty close to the Yagi.

I have a dual receive K3.  If I have both receivers going, one on 
each channel, and I broadcast on one of these antennas, am I in any 
danger of overloading and frying the other receiver?


73, Ed - KL7UW
http://www.kl7uw.com
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