Do you really need 18 ft multiples of coax to tune your antenna?

[Lil'Yeshua]

Well for me a MFJ-259 would be an fairly sized investment. I may have my A99's swr down but being resonant with the modified ground plane kit and Firestik FS99 Fireup top section? Probably not.

 

[Captain Kilowatt]

Using a regular SWR meter there will not likely be a great variety of difference shown by the use of an off resonant jumper in your reading. However, when tuning for more than Standing Wave Ratio, ie, for the resonance of the antenna to a specific frequency utilizing an antenna analyzer, that length is critical.

What is often missed in the discussion of antenna coax length and SWR except by those schooled to the issue/discussion, is that SWR and resonance are not the same, and that SWR may not be an indication of the resonance of the antenna.

It is possible to achieve a low SWR and the antenna be resonant a significant distance along the band away from that low SWR point.
However, when the subject of coax length enters the discussion of antenna tune and performance, the matter of specific lengths must be understrood within the context of what is true or untrue about the influence of those lengths upon what is being read on an analyzer, or what is being transformed by those lengths for things like impedance adjustments, co-phasing, etc.

This is true. The SWR along a transmission does not change however the impedance does. At first this sounds impossible however once one understands that there is an almost infinite number of combinations of R, Xc, and XL that will yield the same SWR it becomes clear. Also resonance is simply the point that Xc and XL are equal and thus cancel each other out leaving a purely resistive component that makes up the impedance. Far too many people think that resonance means 50 ohms exactly. Not so. I have had short low band antennas that were resonant yet had an impedance of only 3 or 4 ohms. I have also had 1/2 wave end fed wire antennas that were resonant yet had an impedance of over 1000 ohms. This is why I like using an antenna analyzer with a 1/2 electrical wavelength of cable to determine just what the actual antenna impedance is that I am dealing with especially if I am making a matching network for it.SWR is only a small indication of what the antenna system is doing. If all you are looking for is minimum SWR than any line length will work as the ONLY time a perfect 1:1 SWR is achievable is when there is zero reactance and the radiation resistance is exactly 50 ohms.

 

[hammer0630]

I recently put up a new Star Duster with 50 feet of Times Microwave LMR-400 and temporarily hooked it up in the basement and had a 1 point nothing SWR needless to say I was very happy about this install so far but rerouting the input from where I wanted to bring the coax in from out side left me short so I bought 10 feet of cable and installed the radio in the house and now had a swr of 1.4 well this was not so good knowing my station is capable of perfect swr's so a few months later I put another 2 foot jumper inline to see what if any difference it would make and my swr's are nearly 1.0 again. So that's my story not sure if this info helps anyone out but I feel better now seeing nearly perfect swr's.

 

[M0GVZ]

Yeah, you really shouldn't be seeing that so it kind of suggests that you have a RF grounding issue.

 

[Lil'Yeshua]

Maybe a rf choke made of RG213 or RG8X near the base of the antenna might help.