On Tue, 30 Aug 2011 16:29:03 +0000 (UTC), "Geoffrey S. Mendelson"
wrote:
Jeff Liebermann wrote:
Take two radios, two j-poles and a lot of coax and make a repeater.
Retch.
Why? Don't you make experimental stuff?
I don't like J-pole antennas. I've also built too many repeaters in
the past to know that they can't just be thrown together from
available parts. One of my friends learned that expensive lesson with
his 2m repeater. He started with two ham transceivers, went to two
Maxtrac radios, and finally to a GE MSTR II. Duplexers went from 4
cavity junk, to a 6 cavity monster.
I did not say it was a permanent
repeater, it's actually a test bed for some repeater/internet linking
systems we are trying out.
Sounds interesting. Plan on everything costing at least 4 times
what's predicted, and for things to take perhaps 4 times as long.
Unplanned repairs and "tweaks" should double the required time.
Budget is zero, but I can swing a roll of rg-6 and maybe a $60 duplexer
if I don't buy anything else (radio/computer related) that month.
Initial budget can be zero. Just plan on a replacement budget equal
to the price of a proper repeater.
How's this for an expensive mistake?
http://802.11junk.com/jeffl/K6BJ-MSF5000/
I've burned about $800 on this mess and it's still not on the air.
Wanna guess what went wrong? (Hint: it involves the internal
duplexer).
http://awapps.commscope.com/products/bsa/_calculators/qhisolation.asp
Hint1: You'll be close with vertical antenna isolation.
Explain, do you mean just the distance if they are both vertical, or
the distance AND one is vertical and the other horizontal?
Assuming vertically polarized monopole antennas, vertical isolation is
from tip of one antenna to the tip of the other antenna. Horizontal
isolation is just the horizontal spacing. If they're seperated both
vertically and horizontally, then there's a problem. You only need to
be offset horizontally slightly, for the antennas to couple. I ran
into this problem when I tried to put two vertically seperated
antennas on opposite sides of a tower. The isolation was better then
they were directly in line with each other, on the same side of the
tower. Incidentally, mounting one antenna upside down offers
additional distance and improved isolation.
Hint2: Some radios have quite a bit of synthesizer noise, much of
which will be on your repeater receive frequency. This is why some
repeaters still use crystal oscillators instead of synthesizers.
Also almost irrelevant. The radios wll be what I have, HTX-202s or what
I can scrounge (TBD).
Retch 2.0. Radio Shock HTX-202 handhelds tend to stick on the air
with low battery voltage. The receivers are also easily overloaded.
We have the remaining few HTX-202 radios used for APRS WX packet. I
would love to replace them, but nobody (including me) want's to spend
the time.
I'd love to get my hands on 2 channel Maxtracs
or similar radios out of taxis, but the people sitting on piles of them are
much more interesting in refurbishing and enhancing them than selling them
cheaply or giving them away.
Patience. On Jan 1, 2013, everything in the US is suppose to go
narrow band. There should be tons of cheap radios available on eBay
and from public safety outlets.
I'd be very happy with ones that receive only or have blown finals that I
can get 2w or so out of.
Make sure you get the ones with the 16 pin accessory connector. Many
of the Maxtrac's on eBay are actually cobbled together Radius or M120
radios, with odd boards and strange firmware. Caveat emptor.
I know, but I don't have a "pair" yet. I'm not even sure there are any
available or a test one. So why be specific when I don't have that information
yet, and why wait for it to ask a technical question, which will have
the same answer no matter which frequencies I use?
Careful. Some repeater frequencies are a problem. For example,
146.760 is on the 41st harmonic of the common 3.579545MHz clock osc
frequency. No problem at the repeater, but it will drive the users
nuts.
If co-located with other radios, be sure to do an intermodulation
study.
http://www3.telus.net/PassiveRF/
We found out too late that a mix of some of our transmit frequencies
lands on one of our inputs.
You can't improve things by simply adding more cavities. All you'll
do is add more loss:
Bigger cavities = higher Q and therefore closer frequency spacing.
More cavities = deeper notch and therefore more isolation.
And that's the answer I need.
You might find it easier to build a UHF repeater. With 5MHz spacing,
the cheap mobile duplexers will work with low power (25w) radios.
--
Jeff Liebermann
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060
http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558