Extra Coax

[KC8FAS]

I have 50 ft of rg 213 coming from my antenna into the crawl space of my house. From there up into the shack through the floor. I have about 20 ft left over coax. Can I keep the extra spooled down in the crawl space or should I keep it in the shack and spool it.


Troy KC8FAS

 

[LtDoc]

It wouldn't make any difference really, so whichever would suit you the best.
- 'Doc

 

[N4KVE]

By spooling it. I assume you mean wrapping it in a coil. That will send SWR through the roof. Sometimes when I have an extra 5 feet in the trunk of my car, I just let it run around the trunk never allowing it to cross itself.

 

[LtDoc]

The only thing affected by coiling feed line is what's on the outside of it, not what's inside the feed line. If there's a change in SWR when you coil the coax then you have a problem to start with, not because the coiled coax caused it.
- 'Doc

 

[N4KVE]

The only thing affected by coiling feed line is what's on the outside of it, not what's inside the feed line. If there's a change in SWR when you coil the coax then you have a problem to start with, not because the coiled coax caused it.
- 'Doc

Sorry, but I politely beg to differ. Years ago I had an extra 20 feet of coax on the run to my antenna, so I coiled it in 12" diameter loops. Checked the SWR, & it was 3:1. I then just ran the coax around the apt with no coils, & it was 1:1. So what could the problem have been?

 

[WA0CBW]

Depending on the type of coax a 12" diameter coil might have been smaller than the recommended bending radius thus deforming the dielectric insulator and center conductor causing a large change in impedance. There could have been other mechanical causes for the increase in SWR. Another possible cause could have been RF current flowing on the outside of the shield. This can upset the way most SWR meters measure SWR. Winding the coax into a coil forms an RF choke changing the current flowing on the outside of the shield thus changing the SWR reading. Transmission line theory indicates that the SWR is constant along a length of coax.
BB

 

[N4KVE]

Winding the coax into a coil forms an RF choke changing the current flowing on the outside of the shield thus changing the SWR reading.
BB

This is what I was taught, which is why I don't coil my coax. Especially when I saw it make my SWR 3:1, but when I undid the coil the same coax was 1:1.

 

[WA0CBW]

Let me explain a little more. Your SWR DIDN'T change. Winding the coil DIDN'T change the SWR only your meters ability to read it correctly. On any given length of coax the SWR is constant. Because you had current flowing on the outside of the coax your meter was unable to read the correct SWR. Bottom line is coiling your coax does not change the SWR according to transmission theory.
BB

 

[k3cfc]

coiling coax

The only thing affected by coiling feed line is what's on the outside of it, not what's inside the feed line. If there's a change in SWR when you coil the coax then you have a problem to start with, not because the coiled coax caused it.
- 'Doc

This is the truth it should not effect a thing. i do it all the time due to the cost of everything.

K3CFC

 

[k3cfc]

Sorry, but I politely beg to differ. Years ago I had an extra 20 feet of coax on the run to my antenna, so I coiled it in 12" diameter loops. Checked the SWR, & it was 3:1. I then just ran the coax around the apt with no coils, & it was 1:1. So what could the problem have been?

Just because you have a 1:1 swr does not mean you have an efficient antenna. you are just matching impedance between radio and antenna. when you coil the coax and the swr went up you are making the coax part of the antenna and this where the interference comes into play.

 

[N4KVE]

Let me explain a little more. Your SWR DIDN'T change. Winding the coil DIDN'T change the SWR only your meters ability to read it correctly. On any given length of coax the SWR is constant. Because you had current flowing on the outside of the coax your meter was unable to read the correct SWR. Bottom line is coiling your coax does not change the SWR according to transmission theory.
BB

Is this because I'm using a $60 SWR meter? Would I get the same results with a good Bird wattmeter?

 

[LtDoc]

It doesn't matter what you paid for your SWR meter, or who made it, they can all be 'fooled' into giving incorrect readings because they can't distinguish between the 'parts' that make up impedance, only the 'lump' impedance. Bird doesn't make an SWR meter, they make a forward and reverse reading watt meter. That is not the same thing at all. I've also seen a Bird meter give false readings because the user didn't know how to interpret those readings, or didn't know why he was getting the readings he was getting. It really isn't always as simple as reading a meter.
- 'Doc

 

[N4KVE]

OK. Thanks. GARY

 

[dksac2]

SWR is a funny thing. You may show 1.2 to 1 at the meter in your shack and have a 6 to 1 SWR at the antenna end of the coax.

I'm newer to all of this, but have been reading everything I can get my hands on and just bought an antenna analizer to help me insure that my antenna gets as much of the 100 watts that my radio puts out.

Do some study, it's all quite interesting and confusing at the same time, but the guys in the previous posts were 100% correct from all that I have read. Now I just have to put it all into practice.

Thanks to those who write so many informative posts, but I advise you still check and don't think every post is 100% correct. Some are and some are not. Do your homework also.

My Best, John

 

[KC4RAF]

You wrote a handful there John about

doing homework. Years ago, when I got started into CBs, the alleged wise ones always said not to coil you coax; if you did it would run up the SWR! Also the myth of 18 foot length for car coax installs. Later on, a fellow ham operator explained and showed me the truth.
After a while you'll learn who to trust with their information here. Several are in this thread, and will try their darnedest to help you learn.

 

[N4KVE]

Also the myth of 18 foot length for car coax installs.

And here all these years I thought that 18 feet was the average distance between a deck lid mount, & a front mounted radio. On my trunk mounted MCS2000, the coax run is 5 feet.

 

[k3cfc]

Some food for thought. if you look up a cb antenna on ebay it will say 18 ft coax. if you look at an antenna for 2 meter vhf/uhf it says 12 ft coax.

 

[LtDoc]

There's no particular electrical reason for that 18 feet length. It's a pretty fair length to reach from here to there in a mobile though. Depends on the mobile and too long is better than too short.
- 'Doc

 

[k3cfc]

I don't know but i tend to follow the manufacture for what they do. I mean they do this for a reason.

 

[KC4RAF]

The 18 feet was the "average"

that was found for most autos and trucks. That is all. It's nothing magical. Get the ARRL book on antennas/coax, it should be there that ANY length will work.