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Re: The QHA antenna.



Jerry,

By saying 'it does not scale':  if you simply scale the dimensions to 435,  you 
do not end up with the excellent omni-directional pattern that has been achieved 
with the 137/144 MHz versions.  There is some additional tweaking required.

However, based on the current NEC input specs that I have from Steve Blackmore,  
for a 435 MHz version,  it should not be too long before there is design that is 
just as easy to replicate as the current VHF version,  and also has an excellent 
omni pattern.

Out of interest,  the 137MHz version that I have built,  was using 8mm copper 
tubing and right-angle connectors.  The tubing is soft enough that it is really 
easy to form the curve, roughly correct on a pipe that has a diameter that is 
close to the helix diameter (200mm or 188mm),  with some adjustment when it is 
finally assembled.

This was much easier than building than the coax versions I have looked at.


-- John Melton  n6lyt,g0orx

        ((
      (|~~|
       `--'
       JAVA

> To: TKVF99C@prodigy.com, John.Melton@uk
> Cc: amsat-bb@amsat.org
> Mime-Version: 1.0
> Subject: Re: [amsat-bb] The QHA antenna.
> Content-transfer-encoding: 7bit
> 
> Steve, John, & the BB,
> Yeah, Steve, we all need another project to keep us busy--hi.
> 
> I've looked at that design, even that particular page, and several others and
> have not thought the QHA would be that easy to build.  This one does appear
> simpler than the more common coax-element ones.  I'm really glad to hear
> several people say they have done it and it wasn't overly difficult.  Even
> Martin, KB2UC, favorably reviews it in his books.  I have been working over
> the last several months on a couple of different designs trying to find that
> elusive "perfect" omni antenna.  Maybe it's been there all along and I was
> just too thick-headed to pay attention.  I usually build a 70 cm prototype
> first since I can easily use the FO-20/29 downlinks as my test range.  I
> recently had a dismal failure of a design I spend several weeks modeling and
> perfecting, only to determine it was far inferior to an eggbeater (my standard
> reference antenna) in actual use.  Even field-strength measurements defy my
> computer models!  I'm begining to suspect the computers are plotting against
> me :-)
> 
> Sorry to be so long-winded, what I really wanted to know is why the scaling to
> 70 cm was a problem.  Is it the thickness of the elements?  Has anyone out
> there built a 70 cm QHA and used it for mode J downlink?  Let us know.
> 
> 73, Jerry




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