[bumstein]

Hey ppl!!

Popping a UHF in my wee zook.. Install went fabulously, or so I thought!!

Stationary and within a few metres of another car I can chat, but it's a bit crappy.. As soon as there is movement or more than a couple of metres, I can hear them fine but they can't hear me...

I'm of the impression that these are more likely to be an aerial/coax thing.. And Im trying to test for short/open circuit, but not really sure how..

If I put the multimeter on the coax cable, I get around 0.5 ohms from the radio end to the base of the aerial on the inside pin and outside thread alike.

However if I test from the inside pin to the outside thread, I still get OL reading. Does this mean the aerial is open circuit and hence stuffed? I thought I'd get back of resistance reading of some description.. Manual for the radio says antenna impedance: 50 Ohms nominal..

I had it installed in another car and it worked brilliantly!! Its been packed up for the last year and a bit..

Any ideas??

Thanks!!

[jimny_timmy]

Did you mount the aerial the same way as you did in the previous car? i.e. on the bullbar?

[swifty_ano]

Mine does this too!!!
Is yours an oricom?

[Samwich]

The GME UHF had these issues in certain models (Not all were effected)

Was told this on the weekend at a UHF course.. Cant remember the models, but we where told GME werent telling people about it, but they will swap them over

[Scrawny]

Sounds like you're on the money with the aerial. Did you solder the plug on or disassemble the aerial at all?

[bumstein]

Jimny Timmy- yep, mounted the same way.. Bullbar mount and earth seems to be good...

Swifty Ano- nope, it's a GME TX 3440...

Samwich- that's interesting!! Might drop an email to GME and see what they say!! Dammit, now I'm hungry and want a sandwich

Scrawn- the plug is soldered and the aerial is more or less intact... It's mounted on a spring and height elevated feed..

Spoke to a mate who's pretty clued in.. He thinks that between the centre and outside pins of the coax should be open... Tested the ground, all ok there... Hmmmmmm this is perplexing!! Have to turn my attention to the radio perhaps

[Samwich]

Also, i had the same issue (But i was getting about 100m before this issue happend)

Found there was a crack on the aerial, screwed it off, new one on.. Now i get about 3kms

[bumstein]

I'm getting an ohm reading from the tip of the aerial to the base/coax.. If it were cracked would it be open?

[Scrawny]

So you soldered the plug on? Are you sure it's done right?

[GeckoXL7]

If you short out one end of the cable and test the OHM reading of the other end should read between 45 - 75 OHM depending on cable.

Also if can test the resistance between both ends of cable center wire should read between 45 - 75 ohm

What areal are you using?

center pin and outer areal should be open depends on areal to, some have a impedance.

Its also recommended esp on 80CH radios that the radio power is connected directly to the battery, if its not you usually get quite choppy signal and interference. I went from just 1k range to 10-15k range when connected direct to battery.

Also communication between a 80CH narrow band and 40ch wide band usually communication is not as loud as a 40ch to a 40ch or 80ch to a 80ch but should still be clear.

[bumstein]

Yeah the plug is right, at least that's how I was shown and there is connection on both the centre and outer wires to the respective plug pin.

Resistance on the centre wire is about 0.5 ohms from end to end. It's a GME AE 4018K1, or something similar I think!! So it has a spring and elevator. I've never dismantled it so at the aerial end I can only test from the nut securing the spring to the outer wire at the other end, and from the base of the aerial to the inner wire at the radio end.

At the moment I have the power wired into accessories so it turns off with the car (so I don't drain the battery). Perhaps wiring it to the battery with a relay on the accessory circuit would work better??

[bumstein]

Oooooh!!! Super fast reply from GME!! Attached below...

Hi Jon

I suspect that the integrated circuit chip that controls the radio's output
has failed. We had a batch of faulty chips between Dec 2009 and Dec 2010 so
if you purchased your radio two years ago then it is likely one of those
affected.

If you can get the radio into us at the address below I can arrange for it
to be repaired under warranty, which normally takes around a week.  

Kind regards,


Could be the problem!! Samwich was onto it!!

[Samwich]

Told you

They werent recalling them cause they are dodgy hahahaha

but maybe check it in someone elses car before you send off? (Someone elses aerial)

[Mattmorgo]

Where's your power supply from I've seen a few sierras now with the exact same symptons from low power supply. Tapping into the cigarette lighter accessories plug isn't enough.

[Samwich]

Run straight off the bat Helps if you turn you car off when wheeling, you can leave the radio on the hear other people haha

[bumstein]

Popped it back to GME... He's pretty confident that that was the issue.. Serial numbers correlate with the ones that had a fault.. They will take about a week to get it back to me..

Dunno about having it permanently hooked up to the battery until I can put in dual battery system... Instead of running it off accessories, I might just pop in a relay and switch the relay off accessories... Id forget to turn the bugger off and have to push start my car every time I used the UHF

[GeckoXL7]

If turn squelch up so don't go of at random with noise there pretty good with power I had mine running from 5pm till 10am next morn and bat was good. A handheld can last 3days continuously on and that's only 3 aaa bats

[christover1]

I left my uhf on in my Moke once, with squelch on, overnight.
Battery was ok next day, but my bedroom was right next to carport,
and I nearly crapped my nappy when I heard sudden loud voices in the middle of the night

[Scales]

glad you got it sorted.

for future reference, measuring resistance on antennas with a multimeter isnt really the way to test them. You need something called a frequency domain reflexometer. And no... i didnt make that up.

Basically it measures the SWR of the antenna against a frequency range so you can tell if its working in the correct freq band. Bit complicated but its the just a fancy way of testing the way different freq's are propogating out from the antenna, as sometimes a break in the antenna conductor doesnt necessarily ruin one. Just changes characteristics.

And Impedence is not the same as resistance, so you wont get the correct reading with just a MM either. in the case of Co-ax you just go pin-to-pin and shield-to-shield with the multimeter (and open circuit shield to pin). If theres a antenna hardwired on the end... its not really possible. One reason why antennas with co-ax connectors are much better IMO.




anywho, enough boffin'in out for me..... If you need radio help fell free to PM me. I was a radio mech in my last job on helicopters.

[bumstein]

Cheers for that fellas!!

Yeah, was reading about the SWR testing.. Had no idea what it was on about haha.. Just trying to look for any indication given the tools and bugger all knowledge that I had at hand

Thanks for the offer scales!! Might just take you up on that!!

Out of interest, any thoughts on shortening the coax cable?? There seems to be different schools of thought on whether this will affect the tuning of the aerial or not... It's hard to stow such monumental lengths in a zook without coiling the bugger (which of course we know we shouldn't do)...

[christover1]

SWR is affected by coax length in 27meg, may be the same for uhf.

With 26meg, length needed to be a ratio of the wavelength

[Scales]

Yeah dont shorten the Co-ax. It is in fact tuned to wavelength to obtain maximum propogation of the signal at the UHF band. If you shorten it, youll affect the max distance you can throw. The cable is actually PART of the antenna, by shortening the cable you are basically doing the same as cutting the tip off your antenna electrically speaking.

Thats the reason 90% of the antennas have the co-ax hard wired to the antenna itself and not have a plug on the end. If they had a plug and left it to the end-user to chuck any length of co-ax on, they would have endless amounts of returns/tech support of people complaining about the performance of the antenna. Oh and dont just cut it and put a disconnect in either. That will affect the impedence of the circuit also and screw shit up at certain freq's. 50ohm is really quite fiddly and accurate.


To tune it properly you need the FDR tool i was mentioning.


Long story short, the average person cannot test antennas, if its not working right and you are sure its the antenna. Its more cost effective just to change it out. or look at your routing.


You should be fine just coiling the co-ax or looping it back on itself, the signal loss in doing so is quite negligble. as long as you arent near any AC sources like your alternator it should be fine. DC doesnt matter as much.

I ran mine on the non-alternator side and kept it away from any EFI sensor lines to be safe (hitting tx will affect the signal on those too a small amount). I suggest doing the same.


Oh and stating the obvious here, but getting a REALLY good earth is paramount on radios. i suggest a fresh earth point and burnish the hole so its a really solid connection. You need to be aiming for 0.2 milliohms here for best performance. Unachievable to measure with a regular ohm-meter so just shoot for 0 ohm between the skin and the lug.




Im a bit of a perfectionist when it comes to radios, so you probably have a bit of breathing room and shit will still work with even the dogiest installations, im just pointing out what a professional install involves on high-end equipment. Up to you how far you go with it.

[bumstein]

Brilliant! Thanks ppl!! Sounds like jolly good advise to me!!

I've got the coax routed on the left side above the wheel arch so not near the alternator or any discernible electrically noisy devises ('93 Sierra) and inside conduit the whole way. Might be able to run a few passes to take up the slack.. Let you know the result when I get the radio back from GME..

[bumstein]

Oh, where do you measure the earth resistance from? Base nut of the aerial to -ve terminal or chassis? Is it worth putting an earth strap from base nut to the chassis so earth isn't relying on ali bullbar attached to front bumper attached to mounts attached to chassis.... ?

[christover1]

bumstein wrote:

Oh, where do you measure the earth resistance from? Base nut of the aerial to -ve terminal or chassis? Is it worth putting an earth strap from base nut to the chassis so earth isn't relying on ali bullbar attached to front bumper attached to mounts attached to chassis.... ?

Making sure bullbar has a good earth makes sense.
I clean and sand of antenna mounting point is probably wise.
If an earth strap is needed from bar to car, wouldn't have to be ran from base.

[bumstein]

Cheers every1!! Your collective wisdom astounds me endlessly!!

If be struggling without you all thanks!!

[fritter]

Wow! I have never seen so much misinformation in one thread.
I know that people are trying to help, but let's dispel a few of the myths.

First some basic info.
The termination at the output of your transmitter is called the load. While the load is most commonly a transmission line to an antenna, it could also be a resistor (often used for testing), your transmitter won't care. Ideally, your load will have the same nominal impedance as the output impedance of your transmitter. So your transmitter, coax cable and antenna should all have the same rated impedance. This will guarantee the maximum transfer of power into the ether and into someones receiver..

Any antenna used at any given frequency performs in EXACTLY the same manner on transmit as it does on receive. The same is true of your coax cable and your connectors. The losses, radiation pattern and gain (if any) are the same.

However, because a poorly matched load will reflect some of the power presented to it, that power has to go somewhere and will be dissipated as heat by warming the coax and the transmitters output transistors. Left unchecked, the transmitters output transistors will eventually go into thermal runaway and be destroyed.
To prevent the destruction of the output circuit due to load mismatch, modern transmitters employ load sensing circuitry that reduces the transmitter power to compensate for the extra generated heat due to mismatch. This will of course result in less power being presented to the antenna, and limit the effective communications range of your equipment.

Still with me?
My advice is buy a good quality antenna, mount and cabling and make sure you have a good ground connection to your mount. This will give you the greatest chance of having a well matched system.

Having stated the above, generally, if you are receiving well, your antenna and cabling are fine.

The myths.
1.

Quote:

If you short out one end of the cable and test the OHM reading of the other end should read between 45 - 75 OHM depending on cable.

Also if can test the resistance between both ends of cable center wire should read between 45 - 75 ohm

If you have a measured resistance of between 45 and 75 ohms on your cable, throw it away. It will be less that 1 ohm unless it's a long run.
The resistance between the core and the outer sheath should be infinite. If not. Toss it.

2.

Quote:

You need something called a frequency domain reflexometer. And no... i didnt make that up.

Didn't make that up huh?

3.

Quote:

And Impedence is not the same as resistance

Please check your facts before posting statements like this. A given impedance can most easily be described as anything presenting a resistance at it's terminals. In it's simplest form this would be just a resistor. Generally however it will be a combination of resistance, capacitance and inductance and ohms law applies. Thus externally, an impedance at a particular frequency looks just like a resistor. That is why impedance is measured in ohms.

4.

Quote:

SWR is affected by coax length in 27meg, may be the same for uhf.

In a properly matched antenna system, coax length will have no effect on SWR. If you have noted changes in the reflected power from the antenna with coax length, then you have a problem with your installation. Most likely your coax has become a part of the radiating element of your antenna.
This is very bad. Not only are you exposing yourself to additional radiation inside your car, you are at risk of RF burns. Even with a 5 watt transmitter RF burns are deep and very painful. If you have seen your SWR decrease with increased coax length, it is because more of the reflected power is being wasted heating coax and not making it back to your meter.

5.

Quote:

Yeah dont shorten the Co-ax. It is in fact tuned to wavelength to obtain maximum propogation of the signal at the UHF band. If you shorten it, youll affect the max distance you can throw. The cable is actually PART of the antenna, by shortening the cable you are basically doing the same as cutting the tip off your antenna electrically speaking.

Not one word of this is true!
Coax cable has extremely high losses. This loss increases dramatically with frequency. The longer the cable run and the higher the frequency, the greater the loss. This is particularly true of the thin RG-58 stuff commonly used in mobile installations.
At 477Mhz a 10 meter length of RG-58 coax cable will have a loss of 3.3db. This means that by the time your 5 watts of power has traveled from the transmitter to the antenna your 5 watts will be reduced to 2.3 watts. Over half of your power has been wasted as heat!
You want all your transmitter power to be radiated by the antenna (ideally), not being wasted warming the cable. Use the shortest cable run that you can comfortably route between the transmitter and the antenna.
This will improve your receive performance as well. Bonus!

There's more but that's enough from me. I hope I didn't come off as being unkind to anyone as that isn't the intention of my post.
Just please don't go giving people advice because you THINK you know the answer. Or you heard something somewhere.
Let's not contaminate the tech.

[Samwich]

Hahahaha my info was legit XD

[bumstein]

Hey all!! Got the unit back from GME yesterday and it works yay

I'm pretty sure I used to get 15-20km range from it, it get about 1km now (having said that, the other vehicle I was testing it with has a pretty crappy and thrashed aerial.. So may be partially to do with that).. I will keep working on it and may just replace the aerial with a new one to see if that picks it up a little or a lot

Thanks for your input Fritter!! Every forum thread google finds has conflicting schools of thought, especially on the "shorten coax or not" conundrum!! But some good info to digest none the less..

Just a query though, because I'm trying to understand as much as possible, and this may be a stupid question undoubtedly!! But you advocate shortening the coax so there is less heat loss, this more signal reaching its destination (or something of the like)? However, you suggest initially that the receiver, coax and aerial should all have the same rated impedance to gauruntee maximum transfer of power..

If I shortened the coax, wouldn't that change the impedance of the coax and causing a mismatched load this overheating the coax and output transistors?? Or am I not inderstanding this right??

[fritter]

Not at all.
The coax cable has a nominal impedance of 50 ohms which is the same as the transmitter output and (hopefully) the antenna.
So the whole setup should be well matched. That 50 ohm impedance the coax exhibits has nothing to do with the cable length, it is an inherent characteristic of the cable itself.
There are antenna designs that use part of the transmission line as a matching transformer, however, these are based on lengths that are a quarter wavelength of line. Pretty small at 477mhz. Also, these antennas generally have a higher impedance than 50 ohms.