High SWR issue

[asuddendeath]

Ford e250 with ladder rack with 1 fiberglass and one aluminum ladder.
Antenna mount drilled right in the center of roof.
Tried a Browning BR-78 and a 4 foot Firestik II. I can get the Firestik around 2.0 but the BR-78 I can not get below 2.4.

Think I have the mount set right.
Are the ladders screwing up my SWR?

 

[Jigglehurts]

How is the coax connected to the mount? What kind of connector ends? 2nd picture under the backing plate, is the Vans metal painted or bare?

 

[asuddendeath]

It's painted. I just checked the ground it looks fine.
Pl259. Try to different pieces of coax. 9 foot and about 6 foot.

 

[Jigglehurts]

The pl259 isn't utilizing the threads on the so239. Or am I seeing differently

 

[Jigglehurts]

Ahh wait those aren't threads. That's the texture on the sleeve that i see. Good picture just my eyes had to double/Triple check that pic to correct what I'm seeing. Sorry.

 

[asuddendeath]

I just scratched off the paint on the underside to expose bare metal for the mount. Even ran a jumper wire from the pl 259 connector to the chassis of the van no difference in SWR

 

[jonwienke]

The PL-259 underneath looks like it might be cross-threaded.

 

[asuddendeath]

It's not. I have removed an attached several times with different pieces of coax

 

[prcguy]

Did you check the match on ch1 and ch40? If so what is the match there? Did you run through the entire range of whip adjustment or did you bottom out or run out of whip?

It would be nice to check it with an antenna analyzer that will give you more information than an SWR meter. I suspect most of the installs for this type antenna are mirror mounts on big rigs with little or poor ground plane. You have a really good mobile ground plane.

When you shorten an antenna with loading coils the feedpoint impedance usually goes down below 50 ohms. It could be your new antenna is in the 25-30 ohm range at resonance and an antenna analyzer will tell you this stuff. An antenna that is resonant but 25 ohms impedance will show a 2:1 match.

Or, if the metal ladder is in the 8 to 9ft range it could be interacting with the antenna, take it off and test.
prcguy

 

[asuddendeath]

ok, thanks for the good info.
Should I be looking for a base loaded antenna then?

 

[lmrtek]

Top loaded like the firestick would be better.
.........
And you may need to add a spring to get the proper swr.

 

[prcguy]

I would buy or borrow an antenna analyzer to see exactly what's going on. Or borrow another type of antenna and see how it plays on your mount.
prcguy

ok, thanks for the good info.
Should I be looking for a base loaded antenna then?

 

[asuddendeath]

I ordered a Stryker antenna. We'll see how that goes. I'll pick up a spring in the next week and toy with it.
Thanks for all your help

 

[asuddendeath]

Okay I think the ladders are really affecting my SWR. I need to raise the antenna. By adding a longer shaft to the antenna how is that going to negatively affect me?

 

[prcguy]

The only way the ladder will affect you is if its somewhat insulated from the roof and its resonant in length. Otherwise its height is so small in wavelengths at 27Mhz that your antenna should not know its there. If your antenna is 4ft high and you had a 2ft high metal thing near it its not like half the antenna is blocked because the signal radiating off your antenna is much larger than the physical antenna size.

Measure the SWR with the ladder on and off to see what happens.
prcguy

Okay I think the ladders are really affecting my SWR. I need to raise the antenna. By adding a longer shaft to the antenna how is that going to negatively affect me?

 

[Ravenkeeper]

As I found out recently with mine, check to see if you antenna (itself, the mast) is grounded. If it is, then you will have a high SWR. I had the insulator in the wrong place, should have had it on the topside of my mount bracket and had it on the bottom.

 

[BloodandRoses]

Those ladders are making your swr screwed up remove the ladders and check swr. The antenna is reflecting off the ladders.

 

[KC4RAF]

The antennas are fiberglass and probably not the cause.

 

[asuddendeath]

Ok good news. Picked up a spring today. around 1.5-2.0 swr. That was just sticking the spring on and not messing with the whip at all.
So it should work out well.
The other thing I might do is get a 18+ inch antenna shaft. Get her up a bit more.
Think the bottom half of the coil was sitting at the top of the ladder and causing reflection.

The other bit of good news is I also picked up a 102 inch whip to play around with when I have a bit of free time. Have yet to set her and see the SWR but im sure it will be fine.

Thanks for all your help.