Why not use the coax to carry switching voltage?
0v for condition 1
+12v for condition 2
-12v for condition 3
and with some ingenuity (designed by ME in 1970's)
12v AC for condition 4
which allows FOUR bands with one wire antenna.
73
Don
N8DE
Quoting Gene Smar <ersmar@verizon.net>:
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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