SWR and Antenna Resadence

[KingCobra_CDX882]

HELLO ALL...

It has been awhile since i have asked or possed a question..

I am sure we all know the lower the SWR of a antenna on a frequency one uses better..

Hopefully most of us also realises an antenna is designed with certian frequencys in mind..

Sometimes the best SWR is not nessesarly where the best radiating of the power from radio and or amp-linear to antenna occures..

So Please explain the differences and why this can happen...

Also....

if a antenna is designed for a frequency and one uses a tuner to use antenna on frequencys outside of antenna design specs..

what would be better.....having an antenna larger then frequency requires or shorter then frequency needs...
or does it not matter at all...

i simply ask out of curosity

do above questions answers differ in use of verticals verses beams..

Later All

 

[949Jake]

I'll give this a shot and I hope others jump in as well.

I am sure we all know the lower the SWR of a antenna on a frequency one uses better..

The SWR at the transmitter only tells you what % of power is being transferred from the transmitter up the feeline to the antenna. It does not tell you how "good" your antenna is.

Something else to think about, at 100 watts a 1.1 SWR means nearly 100 watts is going to the antenna. At a 3.0 SWR, it is about 75!

Sometimes the best SWR is not nessesarly where the best radiating of the power from radio and or amp-linear to antenna occures..

Many antennas are built to be fed with 50 ohm coax and are able to be tuned for nearly a 50 ohm impedance, regardless of the lenghth of coax.

There are some antenna manufacturers or plans that ask you to supply a specific length of coax to feed the antenna. This is done to create a better SWR match. The antenna may or may not be "efficient" and/or resonant.

Another example is if you are feeding an antenna with 450 ohm ladder line, which is virtually lossless, you might see a high SWR at the transmitter. If your antenna is resonant at the frequency, it is not the antennas fault there is a high SWR on the line! In such a circumstance we need a tuner at the transmitter so the we can send nearly full output.

To summarize, SWR is a small indicator of what is happening but it is not the be all and end all of an antenna system.

if a antenna is designed for a frequency and one uses a tuner to use antenna on frequencys outside of antenna design specs..

what would be better.....having an antenna larger then frequency requires or shorter then frequency needs...
or does it not matter at all...

Once again, we get back to feedline. If we are using coax and an antenna tuner, we will have losses with an out of band antenna. The amount of loss will not be worrisome in short runs of coax, but may be in longer runs. With runs of ladder line we will have virtually no losses, but will have to use a tuner.

Longer is better, at least a 1/4 at the operating frequency.

Longer antennas will show some interesting radiation patterns and will have lobes in directions than a smaller antenna wouldn't. But with the increased lobes, you will have nulls too. This might not be a bad thing to you and could make for some interesting contacts.

You can load shorter antennas and it will radiate, but you usually do not want to load an antenna that is not at least a 1/4 wave at the operating frequency.

do above questions answers differ in use of verticals verses beams..

There may be examples that I am unaware of, but in general these principles are pretty fundamental.

For further reading on SWR and transmission line theory I recommend Reflections by Walter M. Maxwell. It is a PDF file. The first section is entitled "Too low a VSWR can kill you," a statement which no doubt would anger some!

I hope that helps. 73