18' of coax??

This is an old thread but it’s always been an interesting subject to me. It’s always amazed me how everyone has a different opinion about coax length. Of course it doesn’t help when antenna manufacturers give out mis-leading information. Basically to answer your question...yes, they are trying to sell you more coax. As far as I can tell, the only time coax length really matters as far as antenna performance, is with the no ground plane antennas such as marine (boat) antennas. I beleave the coax is part of the antenna system. I haven’t heard any good things about these type of antennas as far as performance goes. Of course there’s nothing wrong with cutting your coax to the correct velocity factor length. Especially if that helps you sleep better at night.

 

The other time that coax length is very important, is for Co-phased antennas on your truck. Since the manufacturers are using 75 ohm coax to go to both antennas, they have to be the correct physical length. This creates the 50 ohms impeadance needed to match up to the 50 ohm coax running to your radio.

 

Wouldn't that just require matched lengths and not matched 18' lengths?

 

Yes that's correct. The two antennas need to be 9 ft apart or 1/4 wavelength
apart. The 75 ohm coax is usually rg59a/u, or something like that. The rg59 is usually cut to a certain length which involves calculating in the velocity factor of the coax. The VF is what makes the coax a certain length for the CB band frequency in use. Basically you are taking 2 antennas fed in phase and feeding them into a co-phased harness. So whatever the calculated length of the rg59 is for 1st antenna, you need to cut the rg59 coax the exact same length for the 2nd antenna. I hope this makes sense.

 

Does it matter what coax I use or is one better then another between RG-58,RG-8,or RG-213?

 

Must be that CB has its own RF theory and physics. Truly funny that commercial ops, amateurs and military stations pay little heed to coax length, but CB "gurus" insist that one must have 18 feet. Tell me: If I'm operating at 3.9 MHZ, then I *must have* 60 feet of coax (or thereabouts) IF the CB "experts" are right. So where am I going to PUT 60 FEET of coax in a HONDA CIVIC? It just-ain't-so. IF that old tale were true, then the 7 feet I used to work all around the world with just wouldn't work! But I logged Spain, Russia, the Baltics, France, Germany, Britain, Ireland, Hawaii, and South America on various frequencies from 3.7 to 28 MHZ with a multi-band HF antenna with...…………...7 FEET of coax with an average SWR (not SWR-ZZZZZZZZZZZZZZ) of 1.2-1 on ALL bands. How is CB different?

I concede that phased antennas require certain lengths of coax and a specific distance apart. (BTW...……….Ever hear of ladder line?????

 

Coax has a loss factor that is measured in decibels. RG 213 has the least loss per hundred feet, out of the three you mentioned. But it’s a half inch diameter coax versus the smaller diameter coax like RG 8x, so its not the best choice for mobile use. you won’t notice any difference in performance, since most mobile installations use less than 20 feet of coax. When you buy coax, make sure it’s at least 95 to 97% shielded.

 

but but but .... 18 feet is ... part of the wave length ... right?

The only reason that I recommend 18 feet to someone is this reason ... they have no clue how to put a connector on the end of it and many of these places only sell 18 feet ready to go coax.

I've tried to explain why it isn't that important for "cb" but it is where it matters - UHF and above.

Then we have the folks listening to one "tech" about phase of the signal in the coax and how an insertion phasing is caused when using the wrong jumper and so on.

I have 11 feet with the pickup and do the same thing 80 through 10, no issues at all, but it is 11 because where the antenna is at.

 

Got it thanks

 

RL wrote: "The only reason that I recommend 18 feet to someone is this reason ... they have no clue how to put a connector on the end of it and many of these places only sell 18 feet ready to go coax."

Amen. If I'm home I'll gleefully install my own connectors but if I'm traveling and need a length of coax, 18' assemblies are one of the most popular lengths.