[hackpgh-ham] Group HF antenna build?
- From: Ed Paradis <legomaniac@xxxxxxxxx>
- To: hackpgh-ham@xxxxxxxxxxxxx
- Date: Thu, 9 Dec 2010 09:46:53 -0500
Hi guys and gals,
Now that Matt and Mandy are gone, we can pull out the cool projects.
I mean... err.. umm...
After digging through the interwebs last night and this morning I
finally came upon a website that details how to build a transmitting
magnetic loop antenna:
http://www.standpipe.com/w2bri/build.htm
This is a step by step guide to building a 20 meter magnetic loop
suitable for use up to 100 watts on SSB and ~30 watts on AM or FM.
W2BRI uses 14 feet of copper pipe for the loop and makes a non-tunable
stub capacitor out of RG-8U or RG-213 coax to make a single band loop
antenna. The capacitor and coax feed are attached with hose clamps.
He uses a gamma match to feed the antenna (and gives good instructions
on how to tune the match). I think we have enough parts in the shop
to make one of these as a test.
He has some other tips for magnetic loops on his index page:
http://www.standpipe.com/w2bri/index.htm
If you want a multiband antenna, you have a few routes to modifying this one.
- Make a few different coax stub capacitors and swap them out by
loosening the hose clamps. I think you could avoid tuning issues if
you were careful or came up with some sort of connector system.
- Replace the stub capacitor with a remote-controlled butterfly capacitor.
- You may be able to use relays (or maybe even manual switches) to
swap between a few different permanently attached capacitors
- Just make a different antenna for every band; these antennas are pretty small.
You could probably use an antenna like this from 30 meters up to 10
meters. You might be able to squeeze it to 40 meters but higher than
10 meters and I think it starts acting differently due to its size.
If you're only interested in using the antenna to receive Shortwave or
to monitor the ham bands, check out this excellent build:
http://www.kr1st.com/swlloop.htm
I think that design could be converted to a QRP transmitting loop with
the substitution of a higher rated capacitor and more attention paid
to the feed.
KR1ST also has a QRP transmitting loop based on this design, but there
isn't much write up:
http://www.kr1st.com/magloop.htm
I think I already have all the parts to build one of these.
So this could be another great group build. "Kits" wouldn't be too
expensive. Home Depot says 10 feet of suitable copper pipe would be
between $11 and $38 depending on how thick you want the pipe and
elbows go between $1 and $5 each. Enough coax to build several
capacitors would be in the $10 and then you'd have to buy some coax
and a connector to connect the antenna to your radio. I picked up 50
feet of coax with connectors already on both ends at Radio Shack for
about $20. That could be cut in half, or your could just put a
"pigtail" on the antenna and deal with a little bit of connector loss.
This would put each kit at about $33 and $100 depending on thick the
copper pipe is and how you do the feedline and capacitors.
Ed
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