I havent been able to detect any difference between terminating and floating
the unused direction on a 2 wire reversible. That is not to say that a 50-75
Ohm termination may offer a benefit for TX RF and near area lightning. I
still remember decades ago getting a good nip off a 1000' wire from a storm
that appeared to be about 20 miles away with just distant rumblings.
However I spent considerable time studying and experimenting with binocular
transformers. Success requires as close a match as possible in both
directional modes. This means the parallel wire impedance must be known as
close as possible in transmission line mode. In the antenna mode you start
with the classic 470 Ohms and then subtract your measured RF ground
resistance to arrive at the transformer impedance.
The various books and on line sources appear to insist on only even numbers
of turns with binocular cores when situating a center tap and with the tap
at the same end as the winding start and finish. I disagree and place a tap
at either end as long as its at the center of that particular wire. That
allows much closer matching.
The next step is to isolate all primaries and secondaries to minimize
capacitive coupling. I do this with tubing and a wire size that can make it
thru for the required turns; #26 and 28 solderable enamel works for me.
Altho Ive managed to squeeze 24 in at times the others are easier and loss
difference can barely be measured on a VNA.
And finally you need a stable RF ground. In my case that is 2-3 4' ground
rods driven in at angles at each end with three 20-30' radials attached to
each. Even with all that my RF ground resistance is 200 Ohms confirming that
with solid rock only 1-3' down there is no real ground. As a side note
elevated radials on verticals have worked very well for me.
A completely balanced transformer should give zero output when its opposite
direction is selected and 30-40dB in actuality is more than adequate which
is assisted by having a minimum of capacititve coupling. Isolating antenna
and feedline grounds is also very important.
The end result is F/B that exceeds expectations and remains stable from 4'
of snow over completely frozen ground to totally saturated soil.
Those who live in areas of excellent grounds have different transformer
requirements as well as somewhat less attainable F/B.
Carl
KM1H
----- Original Message -----
From: "Robin" <wb6tza@socal.rr.com>
To: <herbs@vitelcom.net>; <topband@contesting.com>
Sent: Monday, April 18, 2011 12:59 AM
Subject: Re: Topband: 4 Direction Reversible Beverages Remote Control
viaCoax - local switching
> 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
_______________________________________________
UR RST IS ... ... ..9 QSB QSB - hw? BK
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