[TowerTalk] Relay for sloper element

Mon Nov 27 15:00:40 EST 2017

> If I use a DPDT center-off switch on the DC supply in the shack and
> steering diodes in the relay box at the tower top, I can use two
> relays to short out part or all of the loading coil at the feedpoint.
> That way I'd end up with 60M (coil shorted), 75 M (partial coil) and
> 80 M (full coil) on the same wire.
I'm not so sure you will find a single 50 Ohm point over a 1.6:1
frequency range (3.5 - 5.5 MHz).  The so called "half sloper" works
by finding a point roughly an electrical quarter wave from the top
of the tower ... the frequency range 3.5 - 3.9 MHz is not so bad
that a compromise point can't be found.  However, looking to extend
the frequency significantly higher really means the "tap" (attachment)
point should be roughly 1/3 closer to the electrical top of the tower.

Even if one could find X=0 for all three frequencies, the R will vary
considerably and may make matching difficult.

73,

    ... Joe, W4TV

On 11/27/2017 1:53 PM, Gene Smar wrote:
> Jim:
> 
>       Thanks for your suggestion.  If I use a DPDT center-off switch on the
> DC supply in the shack and steering diodes in the relay box at the tower
> top, I can use two relays to short out part or all of the loading coil at
> the feedpoint.  That way I'd end up with 60M (coil shorted), 75 M (partial
> coil) and 80 M (full coil) on the same wire.
> 
>       I now recall seeing this feedpoint loading technique for a half-sloper
> wire (the more correct term) like mine in QST a number of years ago.  I
> hadn't thought of using a relay to make the wire multi-banded.
> 
>       Using a bias-T in this configuration would short the positive DC bus to
> ground in one switch position, so I guess that's out.  Carrying the DC to
> the tower top in a two-conductor-plus-shield cable would isolate the
> positive and negative leads from ground at the tower top, as the relay coils
> would not be grounded in any configuration.
> 
>       Regarding the paralleling of the DPST relay contacts, I'll probably do
> that.  In this configuration (loading coil at the feed) the feed is the
> high-current point and having parallel contacts will minimize the contact
> resistance through which this current must flow.  Also, as you pointed out,
> it provides a bit of redundancy.
> 
> 
> 73 de
> Gene Smar  AD3F
> 
> 
> 
> -----Original Message-----
> From: TowerTalk [mailto:towertalk-bounces at contesting.com] On Behalf Of Jim
> Thomson
> Sent: Monday, November 27, 2017 8:28 AM
> To: towertalk at contesting.com
> Subject: [TowerTalk] Relay for sloper element
> 
> Date: Thu, 23 Nov 2017 12:17:33 -0500
> From: "Gene Smar" <ersmar at verizon.net>
> To: <towertalk at contesting.com>
> Subject: [TowerTalk] Relay for sloper element
> 
> 
> <    I want to be able to switch a piece of #14 house wire onto the far end
> <of a sloper wire I have for 75 M phone so I can operate lower in the band.
> I'm thinking of adding a relay in a weather-resistant box at the sloper's
> end insulator and continue with the extension wire beyond that.
> 
>       My question to this august group:  What relay (and supplier) would you
> recommend for the job?
> 
>       In the alternative, can you point me to a good source of info on
> designing traps for said application?  The trap would not require any DC for
> switching but might take more "cypherin' " to get working.
> 
>       Many thanks and Happy Thanksgiving to all y'all.
> 
> 73 de
> Gene Smar  AD3F
> 
> ##  Ok, this is a quarter wave sloper.   A trap wont work, since you just
> want to be able
> to move the resonant freq down a bit.   The far end is sky high voltage,
> esp with 1.5 kw,
> so  dont add wire at the extreme end.
> 
> ##  Install a spst relay  or a DPST relay... with  contacts in parallel.
> The DPST relay, with contacts in parallel, will provide for doubling the RF
> current capacity and also provides for redundant contacts.
> Use a 12 vdc relay coil, then  feed it with 13.8 vdc  from the shack.
> 
> ##  A small  coil, like with  wound with a total of 4 foot of wire would
> shift the resonant freq down by
> 250 khz.   All the relay does is, shorts the coil out.  A  3 inch diam coil
> has a 12 inch circumference,
> so 4 turns would suffice.  Or use a smaller diam coil and a more turns.
> Install the coil at the feedpoint,
> next to the tower.  A  40 A  sealed spst  automotive type relay  would work,
> as will a lot of other relays.
> The peak V at the 50 ohm feedpoint is aprx   388 volts...assuming 1.5 kw
> into 50 ohms.
> 
> ##  another method is to use bigger gauge wire to begin with.   10 gauge
> wire is a LOT more broad banded
> vs  14 gauge... when using qtr wave slopers.    Im talking about the main
> sloper, not the coil.    14 gauge
> will work just fine for the small coil.
> 
> ##  either  a separate control cable for the 13.8 vdc.... or  a bias T will
> work.    I have used both schemes in the
> past.   If u just want to shift the res point from say  3850.... down to say
> 3600, the above coil + relay setup
> will work just fine.  The default will be the lower freq, with  coil
> de-energized.   If u use a NC relay, then its
> the other way around.
> 
> Jim   VE7RF
> 
> 
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