[Antennas] How to build center feed

Tue, 8 Jul 2003 08:39:20 -0400

Dear Hart,

Couple of points here that don't seem to be well covered by other responses.

The GREAT advantage of the store bought device for constructing a ham dipole
is the fact that the center insulator is usually designed to provide both a
good electrical connection (solder lugs) and a strain relief (steel eyelets
or eye bolts). An then there is the excellent idea of a UHF connector built
into the housing for the coax connection. That means you can buy pre made
cable with the connector already properly connected to it and be able to
connect that cable to the center feed with little concern about strength,
reliability, etc.

One thing that I always add to the configuration is a big gob of duct seal
goop that is sold is small bricks at places like Lowe's or Home Depot. Mash
a gob of that around the assembled coax connection to make it more
waterproof, and it will last for years.

Conversely, a jerry-rigged coax connection at the center feed may otherwise
become contaminated with water seepage into the coax itself - no fun!

Another advantage of the store bought unit is the eyelet on top for hanging.
The weight of the coax line will cause the dipole to droop considerably and
will place more longitudinal strain on the dipole when suspended between two
end supports. Hanging the dipole at its center not only relieves that strain
considerable, but also provides an additional level of control on its bein
level (if that's what you want) or maybe you want an inverted vee
configuration, which dictates a center hanging point.

Regarding impedances - yes, a 75 ohm coax may provide a better match at the
antenna but at the transmitter you still get the mismatch from 75 vs 50 ohms
that you need to match the transmitter designed load impedance, so it is no
big advantage one way or the other to use 75 ohm or 50 ohm coax. In either
case you cannot expect there to be a 1:1 match between antenna, feedline and
transmitter out of the box. It is advisable to plan on using a tuner right
from the start.

Bob Lay in Dahlgren, VA
http://www.qsl.net/w9dmk
----- Original Message -----
From: "Hart Engel" <[email protected]>
To: <[email protected]>
Sent: Monday, July 07, 2003 9:18 PM
Subject: [Antennas] How to build center feed

> I want to build a center feed for a dipole antenna. It should be able to
> connect a 50 ohm coax cable to it. How close should the wires on the
center
> feed be? Or is it better to buy one?
>
> Hart
>
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