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Mobile SWR problems


#1

I’d like to be able to operate mobile on 40m for chasing, especially in between my own activations. The whip I have is quite narrow banded (as expected) but whatever frequency I tune it to I can’t get the SWR below 1.9. I’ve done all the earth bonding I can. I’d like to keep it simple i.e. no tuner, and I have a little linear to give me 30w from my 817. The trouble is, the linear manual says the SWR should not exceed 1.5. Can anyone help with ideas for getting the SWR down? Thanks. 73 John


#2

Hi John,

Can’t help too much but when you said

[quote=“MW0XOT, post:1, topic:12759”]
I’ve done all the earth bonding I can
[/quote] you did try earthing the radio to the car? Just a thought, had the same problem with a friend a while back.

Best of luck

73 Neil


#3

Thanks. Yes I’ve done that, and bonded the roof bars and rails to the car.


#4

I Never been able to get good vswr with roof racks myself.
Measure OHMs with some form of analyser to check how close 1.9 t 1 actually is its probably going to be below 50 ohms . If you can achieve a better vswr with an antenna of a higher frequency on the same mobile mount it could be capacitance back to earth at 40m, it may not be seeing the car body as the only earth in the circuit.
Just a theory or educated guess of course .
I also find my 40m whip on the roo bar is best tuned whilst mobile and laying back in the wind etc if you are going to QSO on the move, I find vswr is different from being stationary too or where you are parked near other objects or out in the clear.
Tricky low bands mobile but good luck.
Ian vk5cz …


#5

Thanks Ian. Good point about the antenna attitude when actually driving. According to my analyser Z is 55 ohms, but I still got X at 96 ohms. 73 John MW0XOT


#6

John,
Mismatch is likely fixable with a matching inductance or capacitance at the feed point.
Best,
Ken


#7

What sort of mobile whip is it you have, like a helical or other ?
Where is the whip mounted, on a bumper bar, bull bar or one of those towball mounts ?

If mounted to close to the car body it may be shunted by the metal work of the car.

Earthing was mentioned, what sort of cable did you use for the earthing bonds. Braided cable is the best for earthing.

A few details and if possible a photo may help.

Like Ian I have found with helical whips mounted on the bullbar, the vswr will vary as the whip leans back and sways about.

I have not mounted a helical whip on my roof bars at the whip is to big and heavy for that.

73’s, Nick


#8

Where is the base of the whip in relation to the roof. Having run HF mobile for many years I find that when the bottom of the whip is below the roof then the peformance of the antenna degrades.

Good luck,

Compton

PS> Is the exhaust bonded to the chassis? It wont be unless you have done it.


#9

Most likely need additional inductance at feed point… try looking at this site which addresses this problem on 40 meters and below:

http://www.k0bg.com/match.html

Really good stuff (all about /m ops…) Good luck and have fun!


#10

Thanks Nick. The whip is an Ampro, which seems to be base-loaded. It’s got 3/8" mount onto a clamp that fits onto a roof bar going across the centre of the car roof. the base of the whip is about 3" above the roof. The clamp, roof bar and roof rails are all earth-bonded with flattened coax braid. 73 John


#11

Thanks for the really useful link 73 John


#12

Thanks John for the further details, I am not familiar with the Ampro whip. It may be the whip is not is at resonace for some reason.
Your earth braid seems ok, I have used RG58 coax with the centre conductor pulled out as a earth braid with good results.

If available a MFJ-259B antenna analyser (or similar device) can be a handy thing to determine where the resonance of the whip actually is.

One minor point is to make sure the 3/8" clamp is electrically bonded to the roof bar. Some roof bars have a protective powder coating which can act as an insulating barrier.

Same for bull bars by the way. Over the years have had a few 4WDer’s confused as to why their UHFCB radios don’t work well.

Many ground independent whips need to be electically earth bonded to work.

That link John provided has some good tips to try out.

Best of luck with getting things sorted out.

Nick


#13

I now run a Hustler, but have used Ampros in the past. Is the resonator rod protruding (intruding might be a better word) a long way into the helically wound base mast? This can cause resonance problems - and a lot of heat instead of transmitted rf!


#14

Thanks Nick. I have a RigExpert antenna analyser which gives me X=96 ohms, which surprised me. I’m going to do some more experimenting having read that really useful web site. You right about the roof bars. Mine are anodised, which is non-conducting, so I had to bond the socket/bracket to the bar. 73 John MW0XOT


#15

Hi Dave. What are calling a long way? 73 John


#16

Actually any distance really. It’s a bit tricky as once you’ve tuned the antenna and then trim any excess off, it can affect the resonant point. Best done in stages. If you’ve got, say, 6" going into the base mast, try lopping off an inch at a time and re-check the resonance.


#17

As you probably guessed, it’s main use is 160 so it needs to be on the short side. Which Hustler do you use, and what SWR do you get?


#18

Are you sure your measuring system is working properly? 55+J96 gives a VSWR of over 5:1.


#19

The Hustler MO-3 straight mast, with the RM series resonators. No more than about 1.3:1 on 40m. As a bonus (they don’t tell you this!) the base mast is resonant on 6m. I’ve worked some quite good Es on it with the 40m coil stuck on top.


#20

Thanks for that. That made me check and I had the analyser set up wrongly! Durhhhh… 73 John