SWR perfect, or faulty....

Thunderpole T600 to a thunderpole orbiter on a 7" mag mount on an aluminium roof rack ( via a steel plate bolted to it before anyone asks ) but also a Sirio 5000PL. Testing for SWR it was perfect, needle slammed right back  but it looked too good, plus I've been struggling to reach people. So, tried a cross needle power SWR meter, the power is going out, but the SWR doesnt even twitch, so no reflected wave coming back. Before I whip it to bits and start testing it, sounds like impedance in the co ax/base? Worth adding SWR on this rig was about 1.3 1.4 when it was installed...

Hi Darren,

It's not everyday that I hear someone complaining about a low SWR.


You mention two antennas and measured them with two meters? Then surely something must be right.

You also mention that you've been 'struggling to reach people'. Does this mean that you've had no contacts whatsoever?
Is there much CB activity in your area? Have you tried driving to higher ground? What channel and times have you been transmitting? Or have you just listened making the same mistake of everyone else who is also just listening no one knowing each other is there because nobody is transmitting? (Does happen too!  )

You also mention an installation SWR of 1.3/1.4 so is all of this after adjusting the antennas?

Just trying to get to the bottom of things rather than offering 'stabs in the dark' guesses. 

I recently combed over one of my radio connectors and settings as I wasn't getting a response from a local repeater on the Ham bands only to realise that the repeater actually wasn't working rather than any problem my end. (Doh!   )

Wanted to make sure that you aren't transmitting out into dead space before scouring for a 'technical' problem that may not be there.


Do you have a friend or contact on CB also so that you can test your setup by transmitting to each other? Even a CB walkie will allow you to check your setup or maybe even a field strength meter. I know that when I first came back to CB I spent weeks 'listening' not hearing a single thing and wondering if there was a problem with my setup. It took a few weeks of transmitting before I got a response and then realised CB was pretty dead in my area. (Hence getting my Ham licence looking for more radio action.)

Anyway, trying to help.

Hopefully there's someone on Charlie Tango that may be in your area to give you a radio check. I hope you get sorted.


All the best,
Victor

Hiya, yes there are people around in Southampton and Winchester only person who got back to me was very local, all the others just didn't really hear me at all and they sounded very vague, not the usual distant signal more like they were in range but whispering in an echo chamber, just not the 'usual' sound. I suspect its the mag mount, it wasn't expensive and the cable entry has got some rust staining ariund the throat into the unit, shame as it sticks like crazy and used to have devent SWR. Second hand walkie is a goid idea...

Interesting this one.
Have you tested your power output.
If you had a fault with the mount or antenna then it would show high swr.
Very strange i will be watching this topic.

Hi Darren,

No worries mate, just trying my best to help out.

I agree with Alan in his posting that this all a bit strange as usually any antenna fault will show a high SWR.

You mention that RF power is going out but do you know if you're getting the full 4 Watts?

It gets more intriguing lol went back out with cross needle gauge, nothing, nothing at all, so tore everything apart, broke down each section and checked it, back together, now 10w of power from a 4w rig and infinite SWR! So broke down each section, tested it with a meter, and the new brand new patch lead was iffy, zero to infinate resistance when you wiggle it, so remade that with new connectors, all good and reality returned So, 3.5w output from a 4w rig, not unusual for this CB apparantly, but still no SWR, no movement of the needle whatsoever but, the last section of co ax lead to the mag mount was 10 ohms, that should be less than 1 surely, about half I suspect, so a mag mount on the way out and the resistance is limiting any
reflected wave which was minimal before anyway at about 1.4 SWR from new? Sound logical?

Nope. Sounds like grasping at straws - can't really test antennas with a DC multimeter. A dead short for example measured with a DC meter may not appear as one to RF and many a perfectly working antenna has been thrown in the bin because the design of the matching section or the balun in it presents as a dead short with a DC meter. A 4:1 balun for example shorts the inner to the outer.

If you've a poor RF ground then less current can flow in the antenna so both transmit and received signals are weaker.

A roof rack provides effectively no RF ground. An antenna install with a poor RF ground will often show very good SWR and often over a wide bandwidth. For mobile CB antennas they're designed with lengths to work well on a poor RF ground because most mobile CB antenna installs have poor to non-existent RF grounds. Measuring 0 Ohms with a DC meter does not tell you you've got a good RF ground, just a good electrical one. You need an antenna analyser to give you an idea - a quarter wave over perfect ground is 36.8 Ohms feedpoint impedance at resonance.

The better the RF ground the shorter the radiating element needs to be for a given frequency, this is also explained in the ARRL Antenna Book section on RF grounding. 

If you've got a good RF ground you usually find you can't shorten the antenna enough with the adjustment available and have to get the dremel out. I use fixed body mount, drill hole through roof method and I bond tailgate and bonnet to the main vehicle body and on something like a Sirio 5000 I'll find out of the box it's resonant somewhere around 26MHz and I have to cut 2-3 inches off.

Thank you for taking the time but I didn't test the antenna, I tested the co-ax and mount leading to the antenna? And I'm not grounding it through the roof rack,  as huge as it is on a Land Rover,  it's going to a mag mount? Now I'm  not an expert, obviously,  but I've been led to believe the mag mount grounds/provides an effective enough ground plane for the antenna to work reasonably well,  which is why I never bothered with a fixed mount. I've managed a verified 12km on it over hilly terrain so it used to work fine?

You think it could be the mag mount?

Is the mag mount like the one in the video as these are known for letting in water via the 4 screw holes and where the cable goes in.




Sirio mag mounts have the same problem as well!

No, mine doesn't have 4 screws, but there is rust staining around the body joint and the cable entry. I concluded it was the mag base so I've ordered the best reviewed one thats actually matched to my antenna, a er, erm, a sirio.....Best grease those screws up then....

The sirio one i got had a badly connected coax in the base.
Common from what i have read but still a good magmount.

Well I've ordered it now so I hope so lol