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Author Topic: Extra Antenna Cable - Appropriate Loop Size?  (Read 441 times)
WA4SE
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« on: May 11, 2003, 08:30:40 PM »

Greetings all!

I finally bought my new mobile (IC-2720, thanks for asking!) and installed it in my car on Saturday.  Since I ended up mounting the radio in the trunk and using a trunk-lip antenna mount, I have a bit of extra antenna cable.  I know I could chop it off and re-solder a PL-259 on the end, but I honestly HATE doing that - I am a HORRIBLE solderer (is that a word?)...

If I bundle the extra wire, is there an appropriate size for the loop?  A quarter wavelength?  A half?  What are the potential problems of leaving it bundled as opposed to soldering a new connector?  I may just have to bite the bullet and put a new connection on if this could cause major compromises during operation...

Thanks in advance!!

-- Scott / WA4SE
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RobertKoernerExAE7G
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« Reply #1 on: May 11, 2003, 09:19:17 PM »

Why not conduct an experiment?

Run the wire out of your trunk forming a BIG circle.

Coil the extra wire.

Run the extra wire around your trunk.

Buy a short piece of coax to go between your antenna and rig.  Either solder PL259s or crimp them on.

I don't have any idea what bands your rig uses.  If it is VHF, note any changes in your vswr.  Find a weak repeater, and listen to it with the different configurations of coiled coax, and the short run of coax.

If it is HF, try listening to WWV, knock the signal down with your attenuater.

Ask a local ham to decrease his power until he is weak copy on your rig.

Get on the local repeater, ker-chunk it a few times, and ask for a radio check (ONLY KIDDING!) ;-))

Play Play play.
Make notes.

Have fun
Bob
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AA8RF
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« Reply #2 on: May 11, 2003, 09:40:13 PM »

A general HF loop size is 8-10 turns 8 inches in diameter though in can vary with cable types and which bands you are on a lot.

-Jim
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N6AJR
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« Reply #3 on: May 11, 2003, 09:45:11 PM »

just dont coil the wire smaller than 6 time the diameter of the wire to keep from causing the center conductor to walk in side the insulation.  I usulay make 6 to 8 inch diameter circles with a little tape holding it as a poor mans balun.. 73  tom N6AJR
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AC5E
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« Reply #4 on: May 11, 2003, 09:57:59 PM »

It gets incredibly hot in cars. Hot enough to melt polyethelene and cause the center conductor to migrate to and short to coax shield. So you really don't want to wind your extra coax too tight.

For the RG58 variants usual on VHF/UHF mobile mounts, a 3" loop is about as small as one should go, and a 4" to 6" loop is better.

For RG8 size (including RG213, etc.) solid poly coax outdoors, you can get by with a 4" loop but 8 to 10" is better for a vehicle. Foam dialectric calls for increasing the loop size by 50 percent for vehicular use.

For the 9913 and equivalent coax I would not care to roll it tighter than 12" outside or 16" in a vehicle trunk or equvalent.

73  Pete Allen  AC5E

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WA4SE
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« Reply #5 on: May 11, 2003, 11:14:46 PM »

Thanks for the replies so far...

I guess I neglected to indicate in my original post, the radio is a dual-band VHF/UHF.  The reason I can't simply run a shorter piece of coax is because one end is permanently affixed to the NMO trunk lip mount.  I will try as some have suggested and make generous coils using electrical tape.  If that is unacceptable, I will bite the bullet and learn to improve my soldering skills a bit.
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N2IK
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Posts: 220




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« Reply #6 on: May 13, 2003, 10:56:47 PM »

Most anyone can solder a PL259 if they have appropriate soldering tools. You need at least a 100 watt iron, not a gun. That will out solder a 250 Watt soldering gun any day. Follow the strip directions in the Arrl Handbook and go at it. Buy a few extra connectors and if you screw it up, chop it off and do it again. New 100 watt irons are pricy. I have seen many used 100 watt irons at hamfests under $10.00. I paid less than $5.00 for mine which works fine. Should you be looking for a new one, American Beauty is a great iron as is Weller. Some like the pyramid shape tips better than the screwdriver or chisel shape.

The archives have some good detailed articles on doing PL259's but doing a few will teach yout the drill. Always measure twice and cut once.

73 de Walt N2IK
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