[Lowfer] Antenna Design (off topic)

Thu, 2 May 2002 03:04:34 -0000

HI Les,

Your biggest problem with a gamma match would be tuning the SWR down
unless you have something like an MFJ259b.
I would suggest 3/4" ID PVC tubing and 3/16" aluminum welding rod.
W1JR has a 4 element design that is very near the size of a 3 element.
It is close spaced, but produces a nice 50 ohm feed. It should be
straight forward to construct and use. If you wish, I will produce the
spacings and element lengths for you. I'll model the pattern for you
for your approval before you build it. I  suspect that you could
use  a direct feed and ignore the balanced to unbalanced problem.
IF it really bothers you, coil some RG58u around the boom as a
choke balun. I think you are correct to not use the FM BCB antenna
as a starting point. The cost will be very low for the 4 element PVC yagi.
The non-conductive boom sidesteps the uncertainty of any boom correction
factor. End mounting of the yagi is preferred. The mast should be about
1/4 wave behind the reflector and should extend above the top of the
reflector
ideally. This may also yeild some additional F/B.

73, Jim Shaffer, WB9UWA.

----- Original Message -----
From: Les Rayburn <[email protected]>
To: <[email protected]>
Sent: Thursday, May 02, 2002 1:53 AM
Subject: Re: [Lowfer] Antenna Design (off topic)

At 22:40 5/1/02 +0000, you wrote:
>HI Les,
>
>If you can tell me more about what you are starting with, I can design a
>proper yagi
>for this frequency.  I will need to know if maximum gain or directional
>discrimination
>is your objective. For example if you have a  station that will QRM you at
>90 degrees
>off heading, a yagi with a null at 90 degrees can be developed. There is a
>very good chance
>that the elements are way too widely spaced and will have to be moved.
>Either more elements
>can be added or the boom may be shortened. If you desire a direct 50 ohm
>feed, then I
>can design something with that in mind too, but often at a slight overall
>performance reduction.
>I need to know boomlength, boom diameter, element diameter, element to boom
>details.
>
>73, Jim Shaffer, WB9UWA.

Jim,

I had first thought about modifying the five element FM beam that
Radio Shack sells, but I think now that perhaps I'd be better off
homebrewing a three element beam.

I need a compromise between F/B ratio and forward gain.

50 Ohm feed would be good, but I think I can manage a
gamma match of some kind. Ideas?

Any help at all appreciated.

73,

Les Rayburn, N1LF
Navy MARS NNN�HSI
"Proudly Serving Those Who Serve"
------------------------------------------------------
4919 Cox Cove
Helena, AL 35080
Try the 1750 Meter Band:
http://www.highnoonfilm.com/xmgr

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