Coax Length Question

Then if your SWR is too high and you don't have time to mess with the antenna, bonding and such, try a different length and see if it won't lower your SWR where you can safely use your radio until you can get everything "perfect"!

 

And if that takes too long, simply unplug the antenna and plug in a dummy load. Instant 1:1 SWR!

 

Easiest way I've found for doing PL-259s (unless you have the proper crimper) is to install a clamp type BNC and use an adapter.

The clamp type has a soldered center pin, and the shield is clamped into place with a compression fitting. Takes me less than 5 minutes, and can be done with a cheap soldering pencil.

 

Yeah but you can't xmit....Say what you want, my radio with a 9ft coax and 3.1:1 + SWR doesn't talk down the road as good as the same set up with an 18ft coax and a 1.6:1 SWR. And my finals are not getting hot with the lower SWR either.

 

So fix your antenna...

The reason you don't "talk down the road as good" is that your mistuned antenna is throwing a fourth of the power back at the radio, and the radio then turns down the power to protect itself.

If your antenna was tuned and matched properly, the length of coax would be irrelevant.

And if your antenna was designed to require a certain length of coax... well, it's a poor design, which wastes a lot of your RF energy heating the coax.

 

i have built many antennas over the last few years, for my HF, VHF, UHF and even for my ole chicken bander radio... coax length has never once made any difference.. once your antenna is properly set and tuned you can add 5 feet or 500 feet of coax and your swr should NEVER change. if it does your problem is in your coax its self. the only time i have ever seen coax matter is on some key down rigs.. i havent got the slightest clue to how those guys set their antennas up but i have talked ot a few over the years and they are the only ones i have ever known to need a certain number of feet for what ever reason.

 

But, AH!!!! You must remember: The coax length thingy is LAW when it comes to the "chicken band". It has special rules and conditions that ONLY apply to CB! CB doesn't follow REAL RF and feedline theory, doncha know! This coax length thing has been preached ever since CB came into existence! I already have many times given the illustration of the 'screwdriver' mobile antenna that covers millions of frequencies with ONE coax of NO particular length. If I wanted to work 1.8 MHZ, IF this were true, then I'd have to have 130 feet of coax in my mobile, or I'd NEVER be able to get my "SWR-zzzzzzzzzzzzzzzzzzzzzzzzz" down! Not only that, for every one of the 10 bands I need to work, I'd have a particular length of coax for EVERY band. Yet, I work from 3.7 thru 30 MHZ with ONE coax about 7 feet long. WHERE would I PUT all that coax?????????? And not ONE, not ONE CB coax length guru, has EVER been able to explain how it is that the screwdriver antenna covers millions of frequencies with ONE coax of random length and does it with an average SWR of 1.2 : 1! It is because coax length (except for some very narrow exceptions) is a bunch of hooey and voodoo electronics!~

The way to set up ANY antenna on ANY radio is to TUNE the ANTENNA, NOT the silly coax! Coax has little to do with it.

When I first started to mess with radio, it was in a military environment. It involved hundreds of frequencies and bands. I never even HEARD of "coax length" until I got involved with CB. And I'm like......."WHAT? ARE YOU KIDDING ME?" I'd been setting up HF radio stations for 6 years before I ever heard something so ridiculous! No, I was taught to tune the ANTENNA, not the COAX.

Sure. You can fiddle with coax, and if it makes your radio work----fine by me. But I want my system to work at optimum. And trimming coax is NOT the way to get an out-of-tune antenna to resonate fully and correctly. Like we said, you can transmit across a 10 watt, 50 ohm resistor, and you'll get 1:1 SWR'zzzzzzzzzzzzzzz" ( I love how people don't understand that there's only ONE "SWR" in a feedline) all day long. It's called a DUMMY LOAD, btw! Lots of good "SWR'zzzzzzzz", but no signal---just like a mistuned antenna! Your "trusted" CB shop LOVES the coax length thingy because it sells more COAX!!!!!!!!!!

GF

 

I hear a lot of talk here by the 'gurus' who never once consider the fact that the antennas resonant frequency is not the trouble. You all lump things together under one heading and fail to ever consider other conditions. Such as the antenna is resonant where you wish it to be, but the lack of image plane reflection or put simply poor ground due to lack of metal in the vehicle is causing the feedpoint to present an impedance too far away from the characteristic impedance of the transmission line. In effect it is an impedance transformation being achieved by conjugate matching whereby part of the line becomes part of the antenna. So the final runs cooler whether or not the ratio of radiation resistance to overall system impedance is optimum. No, every time you only discuss 'resonance' as being the trouble. I suggest if you wish to be a self proclaimed 'guru' you devote some time to study of the mathematics involved. Never have I heard any discuss the difference in conditions when there is a purely resistive termination VS one with reactance. All of you treat the situation the same, as if all setups have no reactance in the system as evidenced by your claims.

There is a reason Heaviside removed quaternions from Maxwell's equations. The reason was engineers and students were too stupid to understand the math in higher dimensional topologies. Problem is as Maxwell well knew you cannot derive the equations for the propagation of radiation in only three dimensions, or four if we add time into the consideration. Tensors and vectors alone cannot accurately describe what is going on in the system no matter how much you insist you know all, everyone else remain quiet. Quaternions in higher dimensional topologies are mandatory if one wishes to understand radio propagation to the level James Clerk Maxwell did.

Always the same old blanket one size fits all conditions claims. Yet most of you think nothing about using your Dentron tuner without ever giving thought to the fact that your bead on things is not so sharp. In the real world you cannot make blanket statements which fail to consider reactive conditions especially since the truck with a perfect ground does not exist. If you disagree, show the theory and show your math.

 

Spoken like someone with a Ph.D in engineering. If you don't have a Ph.D in engineering, please accept my apologies; I don't mean to insult you.

If you go back through my posts, I repeatedly use the phrase "properly tuned and matched". That phrase includes making sure that your antenna system has a decent (I won't say "good") ground or counterpoise as part of the system.

If you want to use lossy, expensive coax as a matching network, be my guest. Personally, I'll set up my antenna properly.