UHF Independent ground Question

Submitted: Sunday, Feb 12, 2006 at 15:27
ThreadID: 30704 Views:4207 Replies:5 FollowUps:10
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G'day, Got a ?? for a UHF expert.

Just fitted a Uniden UH012 to my wifes fourbie, and coupled it to unidens own independent ground antenna. Bull bar mounted

All seemed ok, but before use I thought I'd put an SWR meter on it, as I've got a bit of experience in this area but not an expert, this is where the querie showed up,

its showing full scale reflected power, which under normal circumstances is not good and means a short.

I checked it all out with my DVM and its not a short circuit anywhere from back of radio or antenna.

My question is.... do these new independent ground antennas show up as such on an SWR meter??

If so, why do they come with an Allen key to adjust, and how would you obtain best SWR under such conditions?

I have a matcher aswell if needed but dont think it should be required.

Any help from people IN THE KNOW would be appreciated,

Thanks

Rgds

Ron
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Reply By: Footloose - Sunday, Feb 12, 2006 at 15:41

Sunday, Feb 12, 2006 at 15:41
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 ?)
AnswerID: 154579

Follow Up By: Territory - Sunday, Feb 12, 2006 at 17:57

Sunday, Feb 12, 2006 at 17:57
The needle should not move if the antenna is OK!
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FollowupID: 408551

Follow Up By: Footloose - Sunday, Feb 12, 2006 at 18:06

Sunday, Feb 12, 2006 at 18:06
*Coughs loudly* The forward VSWR scale will most certainly move, otherwise you cannot calibrate it.
Reverse VSWR WILL most definately move. In a correctly matched system it will move to 1.1. This will most certainly will be observeable.
In this case his meter is moving , the problem being why is it showing so much movement ?
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FollowupID: 408552

Follow Up By: Footloose - Sunday, Feb 12, 2006 at 18:17

Sunday, Feb 12, 2006 at 18:17
Incidently, the SWAAR :) meter is only an indication of what's happening. A reflectometer or antannae analyzer set will give quite a few more clues.
SWR meters tend to come in two types, those suitable for HF and those suitable for the higher frequencies. The wrong useage of either might prove to be grossly misleading.
In this case I stand by my question and suggestion.
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Reply By: Darian (SA) - Sunday, Feb 12, 2006 at 15:41

Sunday, Feb 12, 2006 at 15:41
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.
AnswerID: 154580

Follow Up By: Mad Dog (Australia) - Sunday, Feb 12, 2006 at 15:47

Sunday, Feb 12, 2006 at 15:47
no mention because the antenna was pretuned to be somewhere in the ballpark and most don't have a swr meter, swr is very much applicable to uhf.
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Follow Up By: Lone Wolf - Sunday, Feb 12, 2006 at 16:56

Sunday, Feb 12, 2006 at 16:56
Mate, she would have had to put them buggers on AFTER breakfast!
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Follow Up By: Gramps - Sunday, Feb 12, 2006 at 17:55

Sunday, Feb 12, 2006 at 17:55
Naahhh definitely sprayed on !!!
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Follow Up By: Footloose - Sunday, Feb 12, 2006 at 18:10

Sunday, Feb 12, 2006 at 18:10
What I can't get over is the handprints and the tiny writing which appears to say, "G$ramps waz here " :))))))))))))
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Follow Up By: Gramps - Sunday, Feb 12, 2006 at 19:50

Sunday, Feb 12, 2006 at 19:50
I thought it looked familiar. Nothing wrong with your eyesight Footloose:)))))
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Reply By: VK3CAT - Sunday, Feb 12, 2006 at 16:26

Sunday, Feb 12, 2006 at 16:26
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
AnswerID: 154584

Reply By: brett - Sunday, Feb 12, 2006 at 18:39

Sunday, Feb 12, 2006 at 18:39
Might be a dumb question but then it may not. You are using a UHF SWR meter and not a HF one?
AnswerID: 154595

Reply By: Ron173 - Sunday, Feb 12, 2006 at 19:27

Sunday, Feb 12, 2006 at 19:27
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
AnswerID: 154603

Follow Up By: Mad Dog (Australia) - Sunday, Feb 12, 2006 at 20:28

Sunday, Feb 12, 2006 at 20:28
yep sounds like your meter is not suitable. If the radio is drawing the rated current on tx then it's probably ok as radios these days will fold back the output on high swr to prevent damage.
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FollowupID: 408577

Follow Up By: brett - Sunday, Feb 12, 2006 at 21:16

Sunday, Feb 12, 2006 at 21:16
That is your problem, there is probably a good reson why the old 27Mhz swr meters are about $20 and the UHF ones over $100.
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FollowupID: 408586

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