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[aprssig] Re: How Much Antenna for PCSAT?

Stephen H. Smith wa8lmf2 at aol.com
Tue Aug 14 19:05:16 UTC 2007

gerheim at cox.net wrote:
> I've been trying to hit PCSAT with a D7.  Apparently, the rubber duckie isn't enough wire.  Will a telescoping whip do any better?  
> KB9GVR has been having some success.  
> My goal is to send it an APRS packet from a moving motorcycle, so big antennas are out of the question.  I almost managed it back in January.  

If you are referring to the OEM duckie that came with the TH-D7, it is 
little more than a badly-shielded dummy load.  You need something that 
is electrically a half-wave design that doesn't depend on a ground plane 
as a 1/4-wave whip (even full-sized) does.  The problem is that the D7 
is simply too small physically to be a significant ground plane (i.e. 
counterpoise) to most whips.

I have used a dual-band Maldol replacement whip for the TH-D7 (can't 
remember the model number) with great success. This whip is made of 
thin, flexible piano wire (non-telescoping) about 20" long . The base 
and matching transformer is terminated in an SMA connector for direct 
connection to the D7 without the clumsy SMA-to-BNC adapters many people 
use.  The base of the fitting is flared out so that it contacts an area 
on the top panel of the D7 that is about 2 and a half times the radius 
of the SMA socket alone which drastically reduces stress and twisting on 
the lightweight SMA socket on the radio.     This whip radiates about 
10-15 dB more RF than the OEM duck (actually measured on an IFR-1500's 
spectrum analyzer). 

If you are trying to use this for satellite modes, you need to hold the 
radio horizontal and at right-angles to the satellite track so that the 
dipole-type radiation pattern (max is at right-angles to the whip)  will 
extend from one horizon to overhead and then down to the other horizon.


Stephen H. Smith    wa8lmf (at) aol.com
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