The tower I was thinking about using is 110 feet with a pair of 2el 40m
yagis on it. There is also a boom (for a quad) that is on this tower. That
boom is about 25 feet long. The guys are insulated on this tower.
There is the possibility of hanging a T antenna between this tower and
another tower but those two towers are a good 250 feet apart.......that
would mean A LOT of rope to reach to the ends of the horizontal wire and
enough to go up/down each tower. I really do not want to do this because it
is going to be temporary plus would get in the way when more 40m yagi work
will be done in a few months.
IF I do this antenna mounted about 4 feet away from the tower, I have no
choice but to slope the top wires downward.....the angle would probably be
about 45 degrees, perhaps less depending on how far the supporting ropes
would reach. There would be 3 radials on it with 18g wire 10 feet above the
ground with the vertical wire and loading wires 12g.
The two 28 foot sloping wires sounds good to me......I thought they might
have to be longer. Trimming those wires to get the minimum swr to be at
about 1830 will not be easy however. I guess I will have to place myself up
on the tower at 65 feet or so while someone else handles the readings and
the ropes.
Bob NX5M
----- Original Message -----
From: "Terry Conboy" <n6ry@arrl.net>
To: "Robert Pack" <nx5m@txcyber.com>
Cc: <towertalk@contesting.com>
Sent: Tuesday, November 24, 2009 12:59 PM
Subject: Re: [TowerTalk] Variation of Vertical T
> As Wes N5WA pointed out, coupling to your tower could be a real issue. It
> sounds like the height of your tower, plus the capacitive loading of the
> 40m yagi, could make it resonant near 160m. With further info on tower
> and yagi dimensions, it wouldn't be hard to add them to a model to get a
> fairly good idea of the impact. The biggest unknown would be ground
> losses from the tower's connection to earth, if there is a sizable current
> in the tower.
>
> My model of your antenna (without the tower) over "average ground" shows
> about 34 ohms at resonance, using two 28 foot sloping hat wires and two
> 130 foot long, 10 foot high elevated radials (all #14 bare copper). The
> gain is 0.46 dBi broadside and -0.01 dBi off the ends of the wires at 22
> degree elevation.. If the top hat wires are instead horizontal, they only
> need to be 24 feet each and the feed Z rises to 37.4 ohms, with 0.53 dBi
> broadside and 0.09 dBi off the wire ends at 22 degree elevation. There
> isn't much difference, since the hat wires are relatively short in
> wavelengths.
>
> 73, Terry N6RY
>
> On 2009-11-23 9:01 PM, Robert Pack wrote:
>> I have a bit of a dilemma for the short term. While the construction is
>> going on to erect a true elevated 160m vertical (36 foot high base,
>> insulator then 120 feet above that) I need an alternative vertical for
>> 160. My preferred choice at the moment is a top loaded T but current
>> circumstances just are not going to allow me to string up such an antenna
>> between towers. I had to remove the one I had up......or better
>> yet........mother nature removed it for me and I cannot put it back up
>> right now.
>> I do not want an inverted L although it is probably the easiest thing to
>> do....but I do want a vertical. I also do not want to shunt feed a tower
>> and put down miles of radial wire.
>> With a top loaded T antenna in mind.........I put the vertical wire on
>> supports up my 40m tower with the feedpoint 10 feet above the ground to a
>> point at 105 feet up the 40m tower (95 feet of vertical wire) and instead
>> of the top loading being in a T configuration I just slope those two
>> wires at 45 degree angles downward.
>> Does it matter if the top loading wires are really horizontal or can they
>> be sloped as described above?
>> I am assuming the feedpoint will be about 35 ohms so a pair of 75 ohm
>> lines should accommodate that ok.
>>
>> Anyone have any valuable input on this issue?
>> If I can make it work it will be a nice backup antenna for the future.
>>
>> Bob NX5M
>>
>>
>
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