Basic question about SWR...

[hwhitworth]

I have some knowledge of antenna function but am confused about a basic SWR detail. I often see people posting information about adjusting an antenna for low SWR. They speak of shortening or lengthening the antenna to get the proper tune. This tends to give me the impression that the length of the coax has no effect on the tuning of the antenna. Is this true? I would think that from the "transmitter's viewpoint" it could care less whether the length is in the antenna itself or is really a combination of the length of the coax and antenna. Could the tuning of an antenna be done by altering the length of the coax? I am considering assisting a new Ham operator install a dipole antenna at their home and this has bothered me for some time. I know there is a "common sense" answer from an experienced person and appreciate any input.

[5B4AJB]

If you intend to use a dipole with 50Ω coax, you'll need a balun of some kind for a good (about 1.2:1) match.
Length of coax does affect the tuning, but not much.

Here is the classic coaxial balun.
I found an interesting switchable one here, which I might have a go at (for the junk box).

[M1BWT]

It’s normally best to think of tuning the antenna system, not just the antenna – i.e. the feeder, antenna, and potentially anything else in between. In an ideal world the SWR would measure the same at multiple points in the coax, but various factors can influence this.

As 5B4AJB mentions, for feeding a balanced antenna such as a dipole with coax you should really have a balun at the antenna end – failing to do so could potentially lead to currents flowing on the outside of the coax, in which case you’d expect altering the length of your coax to have a more noticeable effect on SWR, as the coax has essentially become part of the antenna. However, the negative effects this brings will be dependent on your friend’s situation (many operate fine without).

When using coax it’s important to avoid certain multiples of wavelengths (to avoid high inductive reactances) – odd multiples of 1/4 wavelength – so it’s worth taking this into consideration.

Assuming you’ve got a reasonable length of low-ish loss coax with a balun, that’s not too close to the wavelength multiple above, altering the length of the coax shouldn’t have a big impact on SWR. Obviously, the less coax the better from a feeder loss perspective!

Ultimately in your case (coax fed dipole) you want your antenna to do all the radiating, with the minimum amount of power being lost in the feeder – concentrating on adjusting the length of the dipole elements, once you're happy your feeder is optimal for your situation. Things are somewhat different when you're using open feeders - some antennas (G5RV) actively make use of the feeder as part of the antenna system, hence length of feeder in that case becomes more significant.

[hwhitworth]

Thanks for the responses. Like most things, the answer can be as simple or as complicated as you want. It's good to learn from others who have more expertise. Hopefully I will be able to assist others in the future. Thanks again.

[KM3F]

SWR: is... the ratio of forward power to reflected power.
When the feedline sees the same resistive impedence from the antenna there should be no reflected power back toward the transmitter or said in a ratio, the SWR is 1 to 1.
Under these conditions the feedline length has no effect except the 'loss' it offers.
.
If the antenna is not the same impedence as the feedline, then the feedline becomes sensitive to length up to the point you are measuring the SWR.
So there are two different conditions you will have and keep seperated in your thinking.
One, a matched condition and two, a missmatch condition.
Again in a matched condition the feedline length has no effect except for losses.
In a mismatch, the feedline become part of the overall match depending on the length and the losses increase as well because reflected power has feedline losses in the reverse direction as well as the forward ditection. Based on this effect, if the antenna missmatch is very large, the losses are also going to be higher.
So bottom line is it pays to have an antenna cut in length to the feedline impedence "AT THE FREQUENCY BEING USED" or make changes in it's length until it is.
An antenna that is close in demension can use a tuner to transform the small difference to the Radio's output impedance.
However, the missmatch between the antenna and the feedline is still there and subject to the same SWR and higher losses between the tuner and the antenna.
Only difference is the radio sees a perfect match looking into the tuner making it output it's full power without any issue.
NOTE; this in not what some people say 'fooling' the radio. It is an impedence transformation from one value to another. Again this is not a fooling function no more than an audio output stage transforming to a speaker impedence, would be.
It's just a function that can be done but not without some side effects when you consider everything that takes place in the whole circuit, even losses in the tuner trsnsformation.
Good luck..