[Hallicrafters] RTMA Dummy Antenna needed for alignment?

[jeremy-ca]

When I worked in R&D developing a 5-450 MHz dual cable CATV based data 
network we evaluated every off the shelf item we could find.

Most barely made it to 30-40 MHz much less 5. What appears to work in a TV 
shop or someones basement is a lot different when looked at on a HP VNA. 
Since phase shift (group delay) was important when running data we wound up 
having Jerrold, RMS and a start up with China connections build to our 
specs.

I also snuck in a spec that splitters, couplers and similar work down to 1.8 
MHz for obvious reasons. Maybe some of those designs found their way into 
general use but what Ive picked up in recent years at the local transfer 
station "pickin' table" are generally the same old junk with rare 
exceptions.

Since it is so simple to build a balun that works exceptionally well for 
cheap it doesnt make sense to settle for less.

Carl
KM1H



----- Original Message ----- 
From: "Glen Zook" <gzook at yahoo.com>
To: "jeremy-ca" <km1h at jeremy.mv.com>; "Roy Morgan" <k1lky at earthlink.net>; 
<hallicrafters at mailman.qth.net>
Sent: Tuesday, April 29, 2008 12:11 PM
Subject: Re: [Hallicrafters] RTMA Dummy Antenna needed for alignment?


> It depends on the TV balun as to whether or not it
> will make it down into the HF region.  Those baluns
> made for use with cable television systems (and most
> baluns available these days are made for use with such
> systems) have to make it down to at least 5 MHz
> because of various signaling that is used in many such
> systems.  Frankly, it has been my experience that most
> TV baluns make it down to the 160 meter band and even
> into the AM broadcast band.
>
> For the article on using TV baluns with amateur radio
> equipment that appeared in Electric Radio a while back
> go to
>
> http://k9sth.com/Page_2.html
>
> The article is the 16th link under the heading
> "Articles".
>
> Although the usual 4:1 matching is not "perfect", it
> has been my experience that using one with the old
> 3-terminal antenna connection receivers usually makes
> a "night and day" improvement in sensitivity when
> antennas fed with coaxial cable are used.
>
> Yes, a separate antenna tuning unit between such a
> receiver and a coaxial cable fed antenna will allow a
> better match between the antenna and the receiver.
> When I first got the Collins 75A-1 receiver (s/n 4)
> going I found that having such a tuner improved the
> sensitivity from absolutely dismal to extremely good.
> However, whenever changing bands (and even within some
> bands) I had to adjust the tuner for best performance.
> Then I tried a TV balun and that worked very well and
> did not require any tuning when I changed bands.
> Depending on the band the improvement between the
> antenna tuner and the TV balun ranged from no
> difference to a very slight difference when measured
> with a signal generator with a well calibrated
> attenuator.  But, considering the fact that the TV
> balun required no physical effort the very slight
> difference in sensitivity was immaterial.
>
> The article describes the effect that using a TV balun
> had on various receivers.  In every case the TV balun
> helped.
>
> Glen, K9STH
>
>
> --- jeremy-ca <km1h at jeremy.mv.com> wrote:
>
> If the input coils were designed for a high impedance
> antenna and a low impedance generator was placed
> across them then the excessive loading would give
> erroneous readings. You can notice the effect when
> using a coax fed dipole; in many cases the signal drop
> is rather dramatic.
>
> If the dummy load isnt available, missnamed since it
> is an early attempt at a broadband matching device,
> isnt available then wind a 9:1 balun on a FT50-43
> ferrite toroid. The old TV baluns are pretty much
> useless below 20MHz if you want accurate readings.
>
>
>
>
>
>
>
>
>
> Glen, K9STH
>
> Website:  http://k9sth.com
>
>
> 
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