UHF Independent ground Question

Hi Ron, no you dont get forward fsd with ground independant. They tune normally. Sounds like something might be amiss, check for continuity. (You didn't accidently reverse the swr meter ? Does it show fsd on forward also ?)

SWR is inapplicable to UHF operation it seems - I've had two and no mention was made in the destructions - all been running fine with good performance - SWR is certainly in the mix forHFCB's .............a different technology all together I'm told. Maybe the techs that follow here can explain why - its all "magic" to me.

SWR stands for standing wave ratio. It is relevant on all radio frequencies. What happens is the Radio Frequency (RF) energy is transmitted via the transmission line (coaxial cable) to the antenna. The antenna is best described as a leaky transmission line that radiates the RF into space. If the antenna is not the correct length for the frequency of use, then some of this RF energy will be returned down the transmission line to the radio. (Think of waves travelling up & down a swimming pool) When the antenna is of the correct length, it is said to be resonnant.

A ground independant antenna means that the input impedance of the antenna should not be affected by the method of mounting - it does not require a ground plane. A typical example is an end fed half wave, ie the 27mHz Station Master antenna. Methods of impedance matching can be capacitice (shows as open circuit at DC), inductive (shows as cloed circuit at DC) or a combination of the two.
Input impedance is matched to the coaxial cable & the transmitter - 50 Ohms.

So, assuming that the impedance matching is OK, then the SWR may be adjusted by altering the antenna length. If the SWR is higher on CH 40 than CH 1 simplex, then the antenna is too long. If the SWR is higher on CH 1 simplex than CH40, then it is too short. Best to tune for the middle of the band - ie CH20

If you can get good reception of other signals, then it is a good indicator that the antenna is resonant. If the reception is good but the SWR is high, it tends to indicate an impedance matching problem. (SWR will be high right across the band)
Likewise, if the antenna length needs to be radically altered from standard (resonnance) then an impedance mis-match is likely. If you can't hear anything, then it is morelikely a transmission line of connector fault - open or closed circuit.
I hope that this is of some help to you & others.


Question. The SWR meter will need to be suitable for UHF - ie good to at least 500mHz. HF units will not work.

If you are in Melbourne, I can check it out for you.
Cheers Tony http://www.qsl.net/vk3cat

Might be a dumb question but then it may not. You are using a UHF SWR meter and not a HF one?

As usual lots of informative replies.

I understand swr and reflected power etc and know that an ideal is 1:1, this is from my old days of cb'ing on 27mhz.

Now possibly here lies the problem, the swr meter I have is an oldie from those days and may not be suitable for uhf?

Although i would have thought that once calibrated on fwd power the reflected power should be same as 27mhz???? were talking RF here, not frequency surely?

There are no electrical shorts, unless my swr meter is tits up. I have another somewhere, will dig that out and check it with that too tomorrow.

From centre pin to shield is infinity throughout the entire set up, even on antenna, so not dodgy antenna.

I have a matcher and can match it in, but its just another box behind the dash which i feel shouldnt be required.

Rgds

Ron