Testing mobile phone anntenae issues

Submitted: Monday, Aug 24, 2015 at 09:41
ThreadID: 130081 Views:1674 Replies:3 FollowUps:1
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I thought I knew what I was doing but it seems not!......

The issue is that we have a suspect antennae connection between the antennae on the bull bar through to the phone..as no increase in signal when in a weak area when in the past there has been

So I placed a multi meter on the fme connector inside the vehicle and got an audio signal with the probes on the centre pin and the outside shield...........so I assumed a short somewhere..........as I assumed that there should be no connection between the two

I know for sure that in performing the same test with a UHF or HF that there should
be isolation between the centre pin and the external braid sheath and so assumed the same would apply with a mobile phone cable?

So in tests so far ....testing an unconnected patch lead....no continuity between the centre pin and braid......same applies to the lead from the spring base with or without the patch lead attached....all good .....BUT connect the whip and there is connectivity between the pin and braid

Disconnect the whip...all silent again...check the whip itself and there is audio between the pin and the thread base.........

Tested another identical set up that works perfectly and once again the same test results of continuity between the points with the whip and not without

So where I am getting this wrong ?.......surely there must be no continuity between the centre pin and the braid for a mobile as the must be in a two radio set up????
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Reply By: Bob Y. - Qld - Monday, Aug 24, 2015 at 10:51

Monday, Aug 24, 2015 at 10:51
Can't help you with the reason for continuity in the phone antenna, Bungarra, but I have a GME phone antenna, AT6DBS, that's never been out of the packet, until 5 mins ago. It exhibits the same as yours!

Maybe the coax has degraded in the old antenna, or the "broomstick" has gone out of tune? We used to find coax would degrade after a few years, where it passed through the under bonnet area, and it was easier just to replace with new. Maybe not so easy with a phone antenna?

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AnswerID: 589593

Reply By: HKB Electronics - Monday, Aug 24, 2015 at 10:54

Monday, Aug 24, 2015 at 10:54
Some antennas will test as a short circuit if they have and inbuilt loading
coil of some description.

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Follow Up By: HKB Electronics - Monday, Aug 24, 2015 at 10:59

Monday, Aug 24, 2015 at 10:59
From mobile one's website:

Most high gain - collinear antennas are DC Grounded and do not need tuning

DC Grounded means the antenna has what is known as a LC Network, Tuning stub or similar method for matching the antenna in order for it to work on the frequency required, what this means is that if you were to measure the connection at the base of the antenna with a ohm meter it would read a DC dead short (DC Grounded) - This is normal and is a common way of designing antennas. DC Grounded antennas do not need a ground plane or earth as in the case of monopole antennas;

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Reply By: The Bantam - Wednesday, Aug 26, 2015 at 09:44

Wednesday, Aug 26, 2015 at 09:44
Appart from gross faults, DC testing is of very little use on radio circuits.

Many of the aerials are either loops or complex circuits rather than simple straight wires and can show DC shorts as normal.

Many of the faults in aerials and feeders may not show DC shorts or bad DC continuity.

In this situation, and not having thousands invested in very specilaised test equipment you are left with inspection and substitution as the only means of fault finding.

It may just be simpler to replace the areial and feeder and ask no further questions.

Many reputable technicians appart from a few quick tests, would not waste their time or your money doing other than replacing the lot.

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