# Re: Cross boom length?

• Subject: Re: [amsat-bb] Cross boom length?
• From: "Michael A. Tondee" <mat_62@xxxxxxxxxxxxxxxx>
• Date: Sat, 18 Mar 2006 19:15:54 -0500

Franklin,
Thank you for your well thought out and informative explanation. Even though
I'm fairly new to the satellite game and the antenna systems involved, I've
been fooling with HF antennas for years. One thing I've learned over time is
that when it comes to antennas almost everything is a compromise of some
sort. Your example certainly proves that. A fourteen foot crossboom is
pretty well impractical so you went with a ten foot crossboom. For me and my
rotator system, ten feet is probably impractical so I'll have to go even
shorter.  I was just curious what a general length was and my question has
I hope I haven't started any debate or anything. I'm just a new kid on the
block trying to find my way and having a blast while doing it.
I guess in a perfect world we would all have all the gain and antenna
seperation we needed to put out optimum quality signals but then that would
take all the fun and challenge out of it wouldn't it? HI HI...
73,
Michael, W4HIJ
----- Original Message -----
From: "Franklin Antonio" <antonio@qualcomm.com>
To: "Michael A. Tondee" <mat_62@netcommander.com>
Cc: <amsat-bb@amsat.org>
Sent: Saturday, March 18, 2006 5:45 PM
Subject: Re: [amsat-bb] Cross boom length?

>
> I have to chuckle at your question, because I know from experience that
> you will get a lot of wildly different answers.  A lot of opinions out
> there.  I'll give you my opinion, and a rationale for it.  Its based on
> what I think is a reasonable model of how an antenna works.
>

> Lets do a thought experiment.  A common 2M circular Yagi made by M2 has a
> spec'd gain of 12.25dBdc.  That's 14.35 dBic.  Lets calculate how big the
> dish of an equivalent dish antenna would be.
>
> gain = 4 pi efficiency area / wavelength^2
>
> I used 145 MHz and 0.55 efficiency.
>
> My calculation comes out 4.5 meter diameter, or 14.8 feet.  The radius of
> this dish is therefore 7.4 feet.  If you mount that even on the end of
> even a 10 foot crossboom, its gonna overlap the rotor and vertical mast.
>
> Now I'm not claiming this is a fatal conclusion.  This is a thought
> experiment based on a simple model after all.  What I do conclude is that
> it is likely that the 2M Yagi does indeed "feel" the mast and rotor
> nearby, to the detriment of its pattern.  For me that means I'd like to
> keep metal as far away from that 2m antenna as possible.  For me that
> meant a 10 foot crossboom instead of a 5 foot crossboom.
>
> A 5 ft crossboom will certainly work, but if you pay a bunch of money for
> fancy coax and preamps to get that last half of a dB here and there, I
> don't think you want to throw away antenna performance by ignoring the
> environment of the antennas.
>
> As I said, I'm sure some will disagree.
----
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