Posted: 1/13/2015 9:26:59 AM EDT
| When mounting radios in the trunk with a trunk-mounted antenna, there's way too much coax from the antenna. Is it better to cut it to length (about 3') or loop it around the trunk and leave it 14' - 16' long? Inquiring minds want to know. |
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Quoted: When mounting radios in the trunk with a trunk-mounted antenna, there's way too much coax from the antenna. Is it better to cut it to length (about 3') or loop it around the trunk and leave it 14' - 16' long? Inquiring minds want to know. I've never modified the antenna wire from a mobile installation/mount.
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I've never modified the antenna wire from a mobile installation/mount. Quoted:
Quoted:
When mounting radios in the trunk with a trunk-mounted antenna, there's way too much coax from the antenna. Is it better to cut it to length (about 3') or loop it around the trunk and leave it 14' - 16' long? Inquiring minds want to know. I've never modified the antenna wire from a mobile installation/mount. Dittos. My radio is literally about a foot from my antenna and i have the 12 feet of cable looping around the trunk lid |
Everything that I have learned in radio installations in the past is to use the correct length coax when possible. Coiling it can cause inductance issues, interference etc. That said, I am not big on cutting coax and putting on new connectors. Its just not a skill I have developed. So I have big loops on my porch and in the truck. 
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Can someone show that there is inductance created or a change in SWR on a coiled coax? You know like invoking science, physics and all that chit.
I was under the impression that a coiled coax is simply a "common mode" choke only functional to that which is present on the outside of the feed line. My guess is completely straight or coiled up coax shouldn't change anything other than removal of current on the shield. Wouldn't the best reason for not coiling be eliminating undue strain on the coax including too tight turns in the coax routing. I am a little tied up with work for a few days but will do some tests with my own gear later with 100' of coax coiled vs straight. |
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Can someone show that there is inductance created or a change in SWR on a coiled coax? You know like invoking science, physics and all that chit.
I was under the impression that a coiled coax is simply a "common mode" choke only functional to that which is present on the outside of the feed line. My guess is completely straight or coiled up coax shouldn't change anything other than removal of current on the shield. Wouldn't the best reason for not coiling be eliminating undue strain on the coax including too tight turns in the coax routing. I am a little tied up with work for a few days but will do some tests with my own gear later with 100' of coax coiled vs straight. Sorry, I said coiled when I meant looped as the OP indicated he was going to do. You are right that coiling it to make a choke would be better. Looping it around in the interior of the vehicle could put it close to other RF sources or current that could cause issues. |
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Yeah I'm not good enough at soldering small connectors to cut anything. I've never had any problems with the coil of wire, no SWR issues and never had any reason to chop off that nice factory connector  I've never bought anything other than a magnet mount with a connector already installed. There's a persistent idea in the CB community that maintains that you have to leave the coax at full length, can't coil it up etc. This comes from antennas not being installed with a ground plane or an insufficient ground plane, and the coax cable shield having to function as a counterpoise. It just illustrates how important the ground plane is to a vehicular antenna, and since it's an issue of size in wavelength, how it becomes more critical as the operating frequency goes down. |
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There's a persistent idea in the CB community that maintains that you have to leave the coax at full length, can't coil it up etc. This comes from antennas not being installed with a ground plane or an insufficient ground plane, and the coax cable shield having to function as a counterpoise. It just illustrates how important the ground plane is to a vehicular antenna, and since it's an issue of size in wavelength, how it becomes more critical as the operating frequency goes down. but my cousin's sister's friend's brother-in-law's step-father installs CBs now at a well known highway truck stop since his plumbing company laid him off, and he told me never cut the coax shorter, ever, because it will mess up your SWRs and blow out the radio! ar-jedi |
OK, here's my first hyper-technical experiment. I ran a SWR scan with the coax coiled and then straight. Here's the test rig.
and the two plots. The heavy line is with the coax coiled and the light trace is with it straightened out. 
I don't have a way to test signal strength, inductance, etc., so this is as good as I can do. |
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About what I would expect. What do you think about the results? Be careful with that shet metal on the table. I have a scratch on my stove from using it as a workbench. I guess I was expecting a bigger difference than I saw. Roger on the table caution. There are a few QST magazines under it to improve radiation and protect the table. |
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Very cool test.
How many times was the test done? (Dataset and standard deviation) If we want to pursue this scientifically. Kind of like shooting two bullets through a newly scoped rifle and calling it good when normally initial zeros are a 5 or 10 shot group each time. I understand that the lines represent 100 samples over the frequency spread but could there be any other possible influences on the test? The reason I bring it up as I had a little time playing with a AA-170 just after I got my AA-54. I was testing numerous HT antennas and discovered my physical proximity to the Antenna could easily sway the results. With HT antennas being not much more than dummy loads I'm curious if a higher gain antenna is more or less influenced by ones physical presence. Also grounding the coax connection at the meter too as an additional test. Hopefully later this week I can do some detailed testing also on a 40m dipole and a bunch of coax. |
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I saw this when I did my clamp mount for the 20m short hamstick. There was way too much stray capacitance and I added a parallel coil to kill off enough of it to tune. Ground means ground...on a vehicle that is CHASSIS. For the tarheel I spent 30 minutes trying to figure out how to get a dremel wand into the back of the body where the mount was going to get the paint off AND added dielectric grease, no issues tuning that bad boy! |