If you try this (good) approach, be SURE you get a PASSIVE video switch. You
do NOT want
video class distribution amplifiers in the path from your beverages, noise,
overload, etc.
will kill you. If the box needs power in order to function, you probably do
not want to
use it. Remember, video folks usually want to use vertical interval switching,
and that
requires active timed solid state active switches.
Some video amplifiers will do OK, but, remember, they think of a millivolt as
SMALL, so a
noise level of 0.1 millivolts is insignificant - 80 dB down to a video person -
but who
wants to listen to 100 microvolts of wideband noise?
I have used some of the small to mid sized 75 ohm type F mechanical switches
intended to
select antennas and cable, some are halfway decent, some are really sloppy. A
back
termination test for what Herb described should be a first test in selecting
such a
switch, if its too cheap to back terminate, you probably don't want it in the
first
place.- and check that it is termination, not just a short to the shield. I
think I
would stay away from the active coax switches used to select between vertical
and
horizontal and pairs of antennas in satellite systems, at least I wouldn't use
them
without extensive testing first. That whole area has good general RF design
and is
usually well shielded, but remember their goal is not ours, and noises at 2 MHz
and at
microvolt levels are below their design radar, as well as really strong
signals. 10 volts
of RF from your own transmitter might let all the smoke out, and 10 volts of
your own
signal on a close in beverage is not far fetched at all.
Robin, WA6CDR
----- Original Message -----
From: "Herb Schoenbohm" <herbs@vitelcom.net>
To: <topband@contesting.com>
Sent: Sunday, April 17, 2011 10:18
Subject: Re: Topband: 4 Direction Reversible Beverages Remote Control via Coax
Feedline
> On 4/16/2011 12:21 PM, Les Kalmus wrote:
>> Walt,
>>
>> Having done just that, here is what I did:
>>
>> I have an Ameritron RCS-8V remote 5 port antenna switch at the support
>> point for one of the Beverages.
>> The other beverage termination is connected to this switch through a run
>> of direct burial RG-6.
>> Unused ports on this switch are not grounded.
> Having read often that common mode rejection and isolation are so very
> important in getting the most out of Beverages I share my experiences:
>
> I have found here that the performance of a reversible Beverage improves
> when the unused port is terminated (75 ohms) improving F/B and noise
> from unwanted directions if that is the case. I gave up on Ameritron
> RCS units for RX antenna switching since there is no provision for
> easily doing the proper termination. I also learned the hard way that
> sending voltages and having switching diodes and having relays reduced
> reliability from enhancing electrolysis (here in the tropics and close
> to the sea) to zillions of ants for some reason being attracted by
> voltage (don't know why) but I have had failures in two DXE boxes
> because of them alone. Trust me they deposit all kinds of crud all over
> the PC board they enjoy living on.
>
> Because good RG-6 is not that expensive I always run a separate
> feedline for each port (direction) and make them exactly 1/2 wave length
> electrical (determined by their velocity factory) into the shack and
> into a surplus (eBay) Dynair video switcher (got them in either PL-259
> or BNC variety). The nice thing about these surplus switchers is that
> *all* unused ports are terminated (until switched on) with internal 75
> ohm resistors. I am assuming that this termination is reflected or
> repeated back at the antenna itself. I test the configuration by
> removing the unused feedline from the box on BC band signals off the
> back of the active two wire Beverage with a significant improvement in
> rejection of signals in the reverse direction. This method seems to
> make the overall performance better and for me was an easy way to get
> away from all the problems associated with switching voltages, stuck
> relays, corrosion, power supplies, and insects.
>
> The Dynair video switch has over 75 db of isolation, is totally passive,
> but the locking push buttons make an audible snap which is a slight
> distraction.
>
>
> Herb, KV4FZ
> _______________________________________________
> UR RST IS ... ... ..9 QSB QSB - hw? BK
>
_______________________________________________
UR RST IS ... ... ..9 QSB QSB - hw? BK
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