Open wire feeders are simple to use if you know to address a few things. They
have a RF field around them so you have to "keep the distant". They usually
have a common mode current as they frequently feed an unbalanced antenna.
In my case, my favourite antenna is a Windom. It is about 200 (maybe 240,
don't remember. Getting old.) feet long and feed at approximately 1/3 from the
end. That leaves a lot of CM current. I use a tuner and wide band choke (my
designs) in the shack right at the point there the feeder enter the shack. The
antenna is used on all bands, 160 - 10 meter. I know it has a "wild" radiation
pattern but that gives the thrill of operation.
The big difference between open and closed (coax) feed line is that you have to
have a low SWR in the closed as you otherwise will lose too much power due to
repeated reflexions in the cable. The open feed allows you to work with almost
infinite SWR (1/10 is about infinite. The feed point on the longwire can reach
5000 ohm and the feeder has a Zo of about 500 ohms) but you still have a lot of
power left on the antenna.
Yes, I know this is the lazy man's antenna as you only have to put it up once,
not take it down for tuning over and over again.
Just wonder what the radiation pattern is for my antenna at the different band.
Anybody can tell?
With best 73 de,
Hans - N2JFS
-----Original Message-----
From: Roger (K8RI) on TT <K8RI-on-TowerTalk@tm.net>
To: towertalk <towertalk@contesting.com>
Sent: Thu, Jun 16, 2016 1:40 am
Subject: Re: [TowerTalk] OWAs or Fans?
Already looked at it. I hate to say it but I'm anti open wire feeders.
Hate 'em with a passion. Every time I've been near one it's caused
trouble. I much prefer coax and a good choke to get rid of common mode.
Typically use your pages that hold a wealth of information, but with the
5 slopers, I see more headaches with open wire than with coax. I'll put
up with a little more loss for convenience. I can still usually join a
pileup and still work the DX on one or two calls, I remember not too
long ago, the station was laughing and said, now there's a ham with
confidence, he only gave his call once, which I had and it worked. I
just listened for a minute or two, saw a pattern after a few contacts,
waited for a lull and gave my call. In most instances, it's timing, not
power. OTOH there are some rock crushers you just wait out.
BTW: They may be over kill, but I think my open, multi core, with
spacing, implementation of your designs will handle the legal limit, key
down, no time limit.
73
Roger (K8RI)
On 6/15/2016 Wednesday 10:58 PM, Jim Brown wrote:
> On Wed,6/15/2016 5:32 PM, Roger (K8RI) on TT wrote:
>> With that set up, it worked well at 200 W, but as I increased the
>> power, around 1 KW (maybe a little less) every LED in the system lit
>> up. Instead of hanging more weight at the feed point, I added another
>> choke with 5 cores and 6 turns ( wound much tighter) where the
>> feedline reaches the tower. That completely cured the problem up to
>> full power out.
>
> Hi Roger,
>
> Look at Slide 17 in k9yc.com/7QP.pdf for a bifilar-wound choke
> connected as parallel wire transmission line. This is a #31 core. The
> choke design is mine, but the slick mechanical design is by Glen, W6GJB.
>
> This choke is good for at least 600W at contesting duty cycles for CW,
> SSB, and even RTTY if installed in a center-fed resonant antenna. For
> greater power handling, a second choke either in the air or farther
> down the line is needed. This choke CAN fry (I've done it) at high
> duty cycles of legal limit CW (calling CQ after you've already worked
> everyone on the band). :)
>
> The wire is THHN, so Zo is 80-90 Ohms; antennas like the C3SS (which
> we really like for portable operation) doesn't like it (I think the
> mismatch screws up N6BT's coupling method), but the C3SS works fine if
> you wind it with a tightly spaced pair #12 enameled wire.
>
> We did something similar in concept with different hardware in
>
> http://k9yc.com/80M-FDVertical.pdf See slide #26. Designing
> conservatively, we used three chokes, but actual operation showed that
> two was enough. Our use was with a KPA500 on CW for 7QP.
>
> You asked about an OWA; I've lost track of what OWA is short for. :)
>
> 73, Jim K9YC
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