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Re: Sat. Antenna Question.



Sid,

Well, you can't just parallel the two antennas in an arbitrary manner and
hope to get a good VSWR. Yes, its a little more complex than that, but not
much. The 1.05:1 ratio was obtained at 50 ohms impedance. Two purely
resistive 50 ohm impedance devices simply paralleled will give you 25 ohms.
This will result in an impedance mismatch (50 ohms source impedance versus
25 ohms load impedance) and result in a high VSWR and who knows what will
happen to the resonant frequency.

Usually, phasing harnesses are used to transform the 25 ohm impedance to 50
ohms. Usually 75 ohm coax is used in specified lengths to accomplish this.
The lengths of the coax are critical. I think this phasing harness is what
you are really missing.

Sometimes phasing harnesses are also used to provide for a 90 degree phase
shift to produce circularity. It sounds like you have already shifted the
yagis in space (hopefully down the longitudinal axis of the boom) by a
quarter wavelength, so you won't need to do it electrically in the phasing
harness. But like I said above, that is only one purpose of the phasing
harness.

To form a 70cm phasing harness, run your main 50 ohm feedline to a coaxial
Tee connector. (DO NOT USE PL-259 TYPE CONNECTORS AT 70cm. THEY ARE NOT 50
OHMS AT THAT FREQUENCY. USE N-TYPE CONNECTORS.) Connect each of your yagis
to the other two ports of the Tee connector with 75 ohm coax of a critical
length. I believe the magic rule is that they have to be the same length,
and each one must be a length equal to an odd multiple of a quarter
wavelength. This electrical wavelength must also take into account the
phase velocity of the particular 75 ohm coax being used. For RG-11 coax for
example, 3/4 wavelength is 13-8/16".

I co-authored an article many moons ago with Keith Berglund - WB5ZDP
entitled "Home-Brew 435 MHz Crossed Yagi - Build This High-Gain Antenna For
Under $20". It appeared in the May 1989 issue of 73 Amateur Radio magazine
and later re-printed in the AMSAT Journal, but I don't remember the issue
date. It provides a very good explanation of  a phasing harness and the
antenna was good for ZRO Level 7 on AO-13. I can mail you a copy of the
article if you like.

I hope this explanation has helped you out. Have fun playing with the
antenna farm there. See you on the birds.

73
Doug - KG5OA

At 12:52 PM 8/1/98 EDT, you wrote:
>I'm new to satellites.  I've built a cross Yagi for 436 mhz and after
trimming
>driven elements each individual yagi antenna gives a swr of 1.05:1 at the
>design freq
>of 436,850 mhz. When paralleled, the yagi combination resonates at 441 (+)
mhz
>with a swr of a little over 2:1.  Spacing of the 2 yagi's was a calculated
1/4
>wave distance of 6-3/4".
>
>Can the swr be improved by changing the physical distance between yagis or is
>it the natural interaction of the two yagis? What do I change to get the
>resonant freq down to 436.850 mhz?
>
>Any help would be appreciated.  Thank you. de Sid AH6HH.
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