Help needed - Mobile CB set up, I can not reduce SWR below 2.5.

Hi as the title suggests I am having issues getting the SWR down, 

I have a Sort wheel base Ford Transit van with a mini beacon and air vent at the rear of the roof a good 3/4 foot from the antenna.

recently I fitted a Thunderpole T800 along with a Thunderpole Orbitor HD antenna, on a mag mount in the centre of the van roof, using a external SWR meter and extending the antennas adjustment up about 10mm in a wide open space the SWR was at its lowest 1.3-1.5 and at the highest 2,  I was happy but the mag mount coax was rooted via a gap by the sliding door where an external work light passes through so was only a temp set up. 

I picked up a body mount and on the weekend and drilled a hole in the roof right where the mag mount was thinking that a body mount would be far better than the mag and help me get the SWR down a little more But NOPE! now my SWR is at its lowest 2 - 2.5 and at its highest 3
body mount has good ground removed all paint inside where it mounts to. 
 

I can not get my head around it! 
and the little adjustment that the Orbitor antenna has little to no effect on the SWR. 

Today I picked up a 105mm quick release and added to gain a little height and although this has very little  noticeable effect on the SWR and I have removed as it looks ridiculous. 

I dont really want to change the Orbitor HD as I drive a lot of areas where it gets knocked around a lot and I don't really want to have to take an antenna off every time it may tickle a tree. 

also having the engine running now seems to push SWR up a bit more and in built up areas its been over 3 well on the way into the red so now I don't want to key up in fear of damaging it. 

Sorry for the long post but hopefully enough info for someone to help or point out what I've been over looking. (a google search found someone else with a similar issue but no resolve)

thanks 
Jack.

Sometimes you can get those body mounts that actually have a terrible ground to the body of the vehicle out of the box due to the construction of the plastic part of it. 
What I would suggest first and foremost, is check the continuity with a multimeter between the outer part of the (I presume 3/8 thread?) socket on the mount and just check the connection from the outer (ground) to the vehicle where you scraped the paint away. And then do the same to again but to the outer part of the PL259 and I would also check the centre of the socket to the centre pin of the PL259. Just to rule out any possible breaks in the coax (even being new). 
I had a body mount that had the same issue and it just wouldn't play ball due to the way it was actually made. I also sent back a boot lip mount with the same issue, the mounting point of the mount where it screws into  the car had no connection to the antenna whatsoever due to plastic and paint. 

But that's what I'd do first, just to make sure you've a good ground. If no joy, I'd be tempted to run a braided copper cable and earth the mount directly from the socket to the van. 

Just a couple of ideas to throw out there anyway. Hope you get it sorted! 

73'

All of the following is assuming no faults, you've not pinched the coax etc....



I'm guessing that the SWR is higher on Ch40 than Ch1 so indicating the antenna is too long? What you're experiencing is exactly what I expect would happen. 

So basically the more efficient the RF ground you have the shorter the radiating element of the antenna has to be for a given frequency - you can read all about that with charts in the ARRL Antenna Book. 

What the issue is for CB mobile antennas is manufacturers make them for a length assuming a poor RF ground because lets face it, most mobile CB installations and in fact most ham HF mobile installs have a poor to non-existent RF ground. 

The problem is that because of it they make the whips a length to accommodate anything from piss poor to non-existent RF ground and there's a limit to how much adjustment they can make available you end up in a situation you have. When you install the antenna with a decent RF ground it then ends up being too long, the resonant frequency too low, and there's not enough adjustment in the antenna to bring the resonant frequency up where you want and drop the SWR. Because you're using a SWR meter that can only test the range of frequencies you can transmit on you can't see that it is actually working, just not where you want it to.

If you use an antenna analyser that can measure outside the CB bands you'll most likely find it's resonant below the CB band. I have decent RF grounds on my vehicles, I install using fixed body mounts and on long antennas like the Sirio 5000 I currently have they tend to end up being resonant around 26MHz. The Sirio 5000 I have I had to cut 3 inches off the whip to bring it up to 11m centred around Ch19 UK FM so I have a low usable SWR over the entire EU CEPT, UK FM and the 10m band where the majority of SSB 10m amateur activity is up to around 28.6MHz. 

I suspect if you beg/borrow/buy an antenna analyser, even those cheap NanoVNAs from Amazon, do a sweep from 24-28MHz you'll find it's working just fine somewhere below the CB band.

26CT1074

Hi thanks for the reply and ideas,
yeah I went over the new coax body mount and connectors before fitting it to the van, then tested after fitting when I ran into trouble.

everything is good and as it should, out of desperation I have once more today gone over it all in my lunch hour, again tested centre of coax to the antenna and outer there is no shorts it has good ground testing via the area the ground meats the metal van roof even tested the outer from both antenna mount point and the outer PL259 (CB end) and a ground bolt almost 2 meters from where the antenna  and its all as it should be. 
 
 73's 

check the coax that came with it as i got mine as was a short on coax, and a few other on here have had the same problems.
i cut 2foot or so of mine resoldered and guess what swr was perfect (there was a break in there)

Northern Crusader wrote:

All of the following is assuming no faults, you've not pinched the coax etc....



I'm guessing that the SWR is higher on Ch40 than Ch1 so indicating the antenna is too long? What you're experiencing is exactly what I expect would happen. 

So basically the more efficient the RF ground you have the shorter the radiating element of the antenna has to be for a given frequency - you can read all about that with charts in the ARRL Antenna Book. 

What the issue is for CB mobile antennas is manufacturers make them for a length assuming a poor RF ground because lets face it, most mobile CB installations and in fact most ham HF mobile installs have a poor to non-existent RF ground. 

The problem is that because of it they make the whips a length to accommodate anything from piss poor to non-existent RF ground and there's a limit to how much adjustment they can make available you end up in a situation you have. When you install the antenna with a decent RF ground it then ends up being too long, the resonant frequency too low, and there's not enough adjustment in the antenna to bring the resonant frequency up where you want and drop the SWR. Because you're using a SWR meter that can only test the range of frequencies you can transmit on you can't see that it is actually working, just not where you want it to.

If you use an antenna analyser that can measure outside the CB bands you'll most likely find it's resonant below the CB band. I have decent RF grounds on my vehicles, I install using fixed body mounts and on long antennas like the Sirio 5000 I currently have they tend to end up being resonant around 26MHz. The Sirio 5000 I have I had to cut 3 inches off the whip to bring it up to 11m centred around Ch19 UK FM so I have a low usable SWR over the entire EU CEPT, UK FM and the 10m band where the majority of SSB 10m amateur activity is up to around 28.6MHz. 

I suspect if you beg/borrow/buy an antenna analyser, even those cheap NanoVNAs from Amazon, do a sweep from 24-28MHz you'll find it's working just fine somewhere below the CB band.

Hey Northern Crusader

mhm No faults found, 
 
what you have said is what I was expecting to happen, but it is the other way around, during another look at everything today my SWR is higher on ch1 
sorry I forgot to mention what channel was what with the readings. 
current SWR readings as follows, 


Ch1 swr 3 / Ch40 swr 2.5  suggesting to short


The antenna is the same length as it came from Thunderpole and I have rased it from 5mm - 10m -15mm thats the limit any more than 15mm and the whip is no longer held by the grub screw.  this has little to no effect. 


Im about to set up a CRT ss6900v with a half wave silver rod as a base station although i have little room for the silver rod it is up I tested the SWR on that and its so low on 1 it hardly moves the meter and about a 1.1 on 40 now Im second guessing if that reading is correct and if the SWR meter is working as should, many thanks for the reply and info I will look at the prices of the NanoVNAs antenna analyser maybe worth it before i pull the rest of my hair out. 

peewee norfolk wrote:

check the coax that came with it as i got mine as was a short on coax, and a few other on here have had the same problems.
i cut 2foot or so of mine resoldered and guess what swr was perfect (there was a break in there)

Hey Peewee, 

yeah I have tested the coax moved it into a number of positions while testing it all seems good, its showing continuity on the core and outer cable but no short between the two. 

thanks for the help

Think about this one as some don't get it.
If your swr meter has knob and switch to set/read to test if it is working correctly reverse it.
Put the coax plugs in the wrong holes and operate the switch fwd/ref the opposite way ie ref to set and fwd to see your swr it should give you the exact same reading as it did using it the correct way round.
Read this again it will make sense lol.

Should have no effect at all Jay.
The only effect with a roof mount is if you put it at the rear or very front it can then be a bit directional.

thanks Alan, 

had less time this morning than I hopped, but the SWR meter reversed as you suggested has the same readings both ways around, so the meter is at least consistent hopefully means its in good working order. 
I did not have time to change the body mount kit. 

I did however grab my Tank whip and put that on just to see what difference It showed from the Orbitor both being very simple antennas just one way longer than the other, out side the house buildings close on most sides, so not the best test conditions,

 results 

2.75m Tank whip = ch1 - swr2  & ch40 - swr1.5
1.55m Orbitor     = ch1 - swr3 (bit over 3 just into the red)  & ch40 - swr2.5 (again a bit over 2.5)

Im defiantly going around in circles here lol 

I have a NanoVNA coming should be here this evening not sure thats going to show me anything new or if ill understand what it tells me at this point.  

been over this setup and tested so many times and not finding a fault is driving me crazy so tomorrow will pull the mount and coax all out start over soldering it all myself.  

Fingers crossed its not raining!

Juast Jack wrote:

I have a NanoVNA coming should be here this evening not sure thats going to show me anything new or if ill understand what it tells me at this point. 

Do a sweep from 26MHz to 28MHz and see if there is a dip in the SWR line anywhere?  If not, widen the sweep until you see one so we know what is going on.

If you connect it to a laptop and run something like VNAsaver, you can also save the trace and post it here.

If it was me i would cut 5mm off the tip and see.
I cut my 4000 down for the 10m band and carry a neutral block with a bit of thick copper wire with me so i can use it on the 11m band.

Alan Pilot wrote:

If it was me i would cut 5mm off the tip and see.
I cut my 4000 down for the 10m band and carry a neutral block with a bit of thick copper wire with me so i can use it on the 11m band.

Yes, but his SWR is high at both ends, and lower on a higher frequency, so either the dip is higher up (and the antenna needs to be longer), or more likely, there is another issue such as grounding….but with a VNA to hand, it’s easier to sweep it and see than chop bits off only to find when you fix the actual issue, it’s too short. 

43CT016 wrote:

Juast Jack wrote:

I have a NanoVNA coming should be here this evening not sure thats going to show me anything new or if ill understand what it tells me at this point. 

Do a sweep from 26MHz to 28MHz and see if there is a dip in the SWR line anywhere?  If not, widen the sweep until you see one so we know what is going on.

If you connect it to a laptop and run something like VNAsaver, you can also save the trace and post it here.

Ok so we see some sun finally on Sunday, I have removed the brought body mount kit (that ive tested 4 times including removing it once already and can not find fault) 

 I have wired in my own self built connectors body mount and coax testing again every thing is as it should be has a good ground no shorts can find no fault and Im getting the same results 

SWR to high ch1 swr3 ch40 swr2.5 only way it goes down is mounting a 9 foot tank whip! 


after a bit of youtube watching ive taken a couple of photos of the nanovna over 2 ranges with a slight change from start to stop. 

Hey Alan, 

Out of desperation although I I didnt think cutting the antenna would reduce my swr I grabbed a length of rod the same length as my whip and cut it down 5mm at a time until i had cut 2 inches off the top and the swr did just keep rising, So i put the original whip back on. 



if anyone knows someone or a business in or around the  Portsmouth / Southampton area let me know im at the stage I need someone else to look at this. lol 

thanks all

Might sound daft this but.
Get a neutral block wire connector thing with 2 screws you know what i am saying.
Then with that wire rod you where cutting add 2 inches to the steel whip and see where that takes it.

Alan Pilot wrote:

Might sound daft this but.
Get a neutral block wire connector thing with 2 screws you know what i am saying.
Then with that wire rod you where cutting add 2 inches to the steel whip and see where that takes it.

will try this if i get time today, 
But as i said in my original post I had fitted a 105mm (4 inch) quick release and seen very little effect on the SWR, would adding extra to the tip have a different out come? 

I know a longer antenna lowers the SWR as ive mentioned I have put the 9' tank whip on it and seen it drop in a usable range though still higher than id like I also cant drive around with a 9' antenna lol 

to me everything pointed to grounding but I truly can not get a better ground without making the van static and driving a massive ground rod right through it into the earth. 

im at the stage a cheap van rig is becoming expensive and almost ready to rip it out and toss it.

Forgot about the quick release.
Put that wire back on you where cutting and cut more off it.
Nothing to loose by taking that extra 4" off.

Ok as it stands I have found a work around kinda.. 

had an old friend look today hes an retired auto electrician hes looked and cant find any faults either says the grounds probably the best hes ever seen on a ford, he's not a CB'er but has fitted a couple in he day so he tells me, he said although the antenna mounts in the centre of the van roof the bulkhead is close and maybe its having a reflective effect. ??

I have finally had a call back from thunderpole, they are not sure either but did say they make and tune antennas on the bases that they are mostly for car roofs and the van could be to much metal and either I would need a longer whip or another type of antenna.. but wanted me to test the antenna on a car to be sure it wasnt faulty, so did that with a mag mount and yep sure thing pretty much a 1:1  SWR on the car roof. 
thunderpole are now looking at what length whip they could supply to lengthen the orbitor and given me  a couple of other options to look into. 

anyway the work around...  my friend in his auto electrical wisdom said stick the mag mount at the rear of the roof 6 inchs away from the mini beacon saying its the worst location for it so sods law says thats where it will work the best... and yep SWR is around 1.5 - 1.6 on ch1 and a swr of 1.2 - 1.3 on ch40!
we fitted the body mount in this same rear location on the estates van where its beacon mounts and have the same low results, I am not sure what other antennas i could try or would cope better on the van.  

but for now im done with messing around and chasing my self around this lol and just want to use the rig so going to leave the mag mount on the rear the SWR is on the very acceptable end without trying to adjust the whip now and Yes the LED beacon will create a lot of noise but only when its on so im just going to have to turn off the CB if I need to turn the beacon on. lol 


thank you to everyone for your responses its been helpful to have ideas bounced around either things id not thought and those I had already looked at, nice to know you are somewhat on the right line of thinking. 


73's

Glad you found away round it but doesn't make sense to me but if it works then that is it.

Juast Jack wrote:

Ok as it stands I have found a work around kinda.. 

had an old friend look today hes an retired auto electrician hes looked and cant find any faults either says the grounds probably the best hes ever seen on a ford, he's not a CB'er but has fitted a couple in he day so he tells me, he said although the antenna mounts in the centre of the van roof the bulkhead is close and maybe its having a reflective effect. ??

I have finally had a call back from thunderpole, they are not sure either but did say they make and tune antennas on the bases that they are mostly for car roofs and the van could be to much metal and either I would need a longer whip or another type of antenna.. but wanted me to test the antenna on a car to be sure it wasnt faulty, so did that with a mag mount and yep sure thing pretty much a 1:1  SWR on the car roof. 
thunderpole are now looking at what length whip they could supply to lengthen the orbitor and given me  a couple of other options to look into. 

anyway the work around...  my friend in his auto electrical wisdom said stick the mag mount at the rear of the roof 6 inchs away from the mini beacon saying its the worst location for it so sods law says thats where it will work the best... and yep SWR is around 1.5 - 1.6 on ch1 and a swr of 1.2 - 1.3 on ch40!
we fitted the body mount in this same rear location on the estates van where its beacon mounts and have the same low results, I am not sure what other antennas i could try or would cope better on the van.  

but for now im done with messing around and chasing my self around this lol and just want to use the rig so going to leave the mag mount on the rear the SWR is on the very acceptable end without trying to adjust the whip now and Yes the LED beacon will create a lot of noise but only when its on so im just going to have to turn off the CB if I need to turn the beacon on. lol 


thank you to everyone for your responses its been helpful to have ideas bounced around either things id not thought and those I had already looked at, nice to know you are somewhat on the right line of thinking. 


73's

I havent read the thread through, however just to add I run my cb from my magmount on the bonnet of my VW T4. 1.1 swr and works great. Have your tried the bonnet?

Alan Pilot wrote:

Glad you found away round it but doesn't make sense to me but if it works then that is it.

yeah I am none the wiser either, but if it works it works and ill take that.

hey Jack,
i got a siro and on my extra long, high top van got a swr of 1:1 on nearly every channel, after a bit of help from Alan, my coax had a short 2" in as there are quite a few on here had the same prob, sometimes was ok even with analiser...... worked great on the van...... fast forward 4 months, new job, no van.... now a mini and yes i had to ajust the whip by around 2" but swr 1;1 on every channel now, carnt beat a siro to be honest as you drive around just look at what is on cars , vans and even wagons most use a siro either 4000, or the slightly more expensive 5000,
glad you are sorted now, hope to catch you on air some time
73`s

Thanks Peewee, 

Just looked at the Sirio line up, the 5000 with the adjustable joint and locking handle the ability to lean it back when going places it may get knocked about, that maybe a worth while upgrade later down the road. 

anything on the roof tends to take a lot of abuse, I have to take the van into some wild places lol  

73's