[amsat-bb] Re: Station not coming together - the full post (sorry for repost, reply to this)
jonny290 at gmail.com
Wed Mar 7 16:15:37 PST 2007
Yeah, I think the next paycheck's fun money is going to an AMSAT membership
and a stack of 1/4" aluminum rod, heh!
I'm designing the beam on paper, and as per your suggestion, I'm attempting
it with a single beam. So far, it looks great - folded dipole element, about
11db forward gain, exactly 50 ohms impedance, great bandwidth and at 30
degrees elevation, the horizon is 1.5db off peak response, and the -10db
response on the upper side should be around 75 degrees.
I recall that satellites spend only an infinitesimal amount of time above 75
or 80 degrees elevation, and though it'd be ironic that I'd lose the
satellite directly overhead, when it's at lowest path loss, I can deal with
that for 30 seconds in one out every 20 passes I work, by my estimation.
If the single one works and I want to improve upon it, it'll be trivial to
build another and phase them up.
Question:, for linear polarized antennas, am I going to see any benefit by
orienting it any particular direction? I ask because I remember that
horizontally polarized signals 'bend' a little bit more over the horizon,
though admittedly this effect will be very small at 435 MHz; I also believe
most man-made noise is vertically polarized, so I'm thinking that keeping it
horizontal would be the best course of action.
Should I go Arrow-style and design a 3 element 2 meter beam at right angles
to the 435 beam? If the more important 70cm antenna is horizontal, this
would put the 2 meter beam vertical - and even though it'll be at a 30
degree elevation, that would give the uplink antenna a little bit of
'downtime' utility as an FM simplex antenna. That's a very minor issue, but
I like to lay it all out as it zips through my head - due to my QTH
constraints, antennas that can do 'double duty' are especially valuable to
I suppose I could just buy an Arrow....but that's the easy way out (not that
those aren't great antennas!)
The Yaesu 817 is a great radio, but I am an irrational brand loyalist for
some reason, and I can't help but be obsessed with Icom gear. My big
decision is "435 downconverter or Icom 706mkiig" this spring. The
downconverter would come about $700 earlier, but the 706 would give me the
all-important all-mode 70cm receive as well as 70cm all mode transmit if I
want to play with VO-52 or AO-7 in B, etc. My 735 is a hot radio and I
believe it'd work very well with the downconverter and an ARR preamp, but
the 706 is very tempting.
If the IC-703 had 70cm coverage, I'd have picked one up three months ago.
It'd be perfect for this application - I wish they'd realize that and add
it. They'd gain a lot of satellite operators with a rig you can snag for
$450 or so - and it seems as though you can get by on 5 watts usually!
On the downconverter - mast mounting - good idea, bad idea? I see the upside
of "28 mhz cable on RG213 = good", but the downside of "environmental
factors/temperature on a crystal oscillator = bad." Thoughts? I'm confident
that I can weatherproof, but I'm mainly concerned about the temperature
Well, I think that's all for this evening - back to antenna building :)
Matt (or Jonny, whichever :) )
More information about the AMSAT-BB