Joe:
Thanks for your note. I am prepared to fool around with the wire
and/or coils on the tower before I throw up my hands and try Plan B. You're
probably right about this being a difficult engineering problem to solve in
the real world, but I'll give it a shot. Recall that my first idea was to
add a wire with a relay to lower the resonant freq. I'll try that first.
I'll cross my fingers and hope the rig's internal tuner can handle the
mismatch on the lower freq. without moving the tap point on the tower.
73 de
Gene Smar AD3F
-----Original Message-----
From: TowerTalk [mailto:towertalk-bounces@contesting.com] On Behalf Of Joe
Subich, W4TV
Sent: Monday, November 27, 2017 3:01 PM
To: towertalk@contesting.com
Subject: Re: [TowerTalk] Relay for sloper element
> 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@contesting.com] On Behalf Of
> Jim Thomson
> Sent: Monday, November 27, 2017 8:28 AM
> To: towertalk@contesting.com
> Subject: [TowerTalk] Relay for sloper element
>
> Date: Thu, 23 Nov 2017 12:17:33 -0500
> From: "Gene Smar" <ersmar@verizon.net>
> To: <towertalk@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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