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SWR drastically lower when mount wet

Joined
Dec 27, 2007
Messages
553
Location
Central AL
I'm trying to install an antenna on my 01 F-250 but can't really get below a 2 for my SWR. I'm using a Firestik bed stake mount in the hole closest to the cab of the truck. The antenna is a 4ft tunable tip Firestik.

Strangely enough, when I measured the SWR after a heavy rain with the mount wet it was down to 1.3-1.5. When I dried the mount off it jumped back up to just over 2. So I poured a little water on the mount again and it went down again. What would cause this? I have continuity between the bed and the mount so I wouldn't think it's a grounding issue but I'm unsure.
 

KEWB-N1EXA

Member
Joined
Jan 1, 2020
Messages
240
Location
Acushnet Heights New Bedford,MA
I'm trying to install an antenna on my 01 F-250 but can't really get below a 2 for my SWR. I'm using a Firestik bed stake mount in the hole closest to the cab of the truck. The antenna is a 4ft tunable tip Firestik.

Strangely enough, when I measured the SWR after a heavy rain with the mount wet it was down to 1.3-1.5. When I dried the mount off it jumped back up to just over 2. So I poured a little water on the mount again and it went down again. What would cause this? I have continuity between the bed and the mount so I wouldn't think it's a grounding issue but I'm unsure.
The water is making a better Conductive ground for the antenna.
The issue is your thinking in a DC Ohm meter world ...I see low ohms so its grounded !
The antenna is working in an AC environment .. You have an Impedance issue and adding water is changing the value.
I think your best option is to use a small CB antenna tuner to fix your impedance issue tuning the coax impedance to match your antenna. That or wash your truck every time you go to use the radio ! Humor ....

I'm going to stand back now because the locals are coming in on the express train with their opinions !

Peter N1EXA
 
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Joined
Dec 27, 2007
Messages
553
Location
Central AL
Take a picture of the antenna so we can see it's surroundings.
I've got one attached. There's a different antenna in the picture that I was using to test but you get the idea. It's close to the cab but it seems like I see tons of installs done this way so there's got to be something wrong with mine in particular.
 

Attachments

Joined
Dec 27, 2007
Messages
553
Location
Central AL
Is the antenna base insulated from the truck bed by the bed liner?
No, you can't really see it but there is just enough clearance around that hole where the base is able to make full contact with the metal bed. Fits like a glove actually. I have continuity between the bed and the mount.

Note particularly item 3 at Troubleshoot a High SWR
I saw that but I also see lots of trucks with antennas in this same spot. Surely they aren't all riding around with a high SWR but I don't know.

I'm still wondering if I've got grounding issues. I DO have continuity between the antenna mount and the bed but none between the mount and the negative battery terminal.
 
Joined
Dec 27, 2007
Messages
553
Location
Central AL
A few more observations. With everything dried out I observed lower SWR readings (under 2) when I used my SWR meter and radio to test instead of my NanoVNA. I confirmed with another setup that the NanoVNA and SWR meter should be giving the same readings, but they aren't with this particular setup.

The only explanation I can see for that is the NanoVNA is powered by a battery so nothing is connected from it to the truck battery/grounding but when using a radio and meter to test SWR, the ground from the radio is also connected to ground. Not really sure what to make of that but I'm going to try adding a few grounding straps to see if that changes anything.
 
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kc4jgc

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Joined
Mar 7, 2004
Messages
1,481
Location
Virginia Beach, VA
I agree with slowmover. In my view, it's too close to the cab. Steel from that is detuning the antenna. Why the rainwater is making it better is odd though.
 

cmdrwill

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Mar 30, 2005
Messages
3,893
Location
So Cali
BIG Red flag: SWR on CB radio shows "good" and with real test equipment, shows bad. No RF or DC ground.......
 
Joined
Dec 27, 2007
Messages
553
Location
Central AL
Seems like I've founding a working solution. The antenna I first tried was a 3ft, followed by a 4ft, and finally a 5.5ft. None of them had an SWR below 1.8-2. On a whim, I decided to try this old 96" fiberglass antenna from my spare parts pile. Over the years I've wound up with quite a few spare antennas for CB and VHF that I keep on hand since they don't take up much room. I have no idea who manufactured this particular antenna but it's old, probably from the 70s or 80s and is lower profile fiberglass with a diameter that tapers smaller the further up the antenna you go (rather than being one thickness the entire length like a Firestik).

Tried it out in the same mounting location and sure enough the SWR for it is flat across the band, only going up to about 1.3 on Channel 40. Everything else is 1.0-1.1. With a spring the SWR pretty much stayed the same. I wasn't really wanting to use an antenna quite this long but it works and it was free so I'll go with it and return the other stuff I bought. I figured my wife wouldn't like it but her response was "Well, it's a big truck so a big antenna looks fine on it". Works for me!

I wanted to stick with this particular mounting point because it was really the only one that made sense for me. I didn't want to use the rear-most stake pocket or the bumper because that would get in the way of hauling things in the bed or using the fifth-wheel hitch. I didn't want to put something on the roof because it would be harder to climb up there and unscrew the antenna if I needed the clearance. And aesthetically I don't really care for front-fender mounted antennas. So this was really the best mounting point for me.
 

KB4MSZ

Billy
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Joined
Mar 12, 2018
Messages
546
Location
Tampa, Florida
I assume less interaction with the cab due to the antenna having a more clear path. I have a unloaded 102" whip that is also behind the cab directly on the back bedrail. It provides good coverage of both 10 and 11 meters.
 

FiveFilter

Member
Joined
Jan 1, 2016
Messages
252
Even though the lower part of the antenna carries a substantial amount of the total energy emitted and is still blocked somewhat by the cab in some directions, it is apparent the long antenna overcomes this to a satisfactory degree.

However, I suspect that testing would still show some degradation of the signal, particularly forward of the truck.

In any case, life is full of compromises, and this seems to be a good one.
 
Joined
Dec 27, 2007
Messages
553
Location
Central AL
I assume less interaction with the cab due to the antenna having a more clear path. I have a unloaded 102" whip that is also behind the cab directly on the back bedrail. It provides good coverage of both 10 and 11 meters.
That's my theory as well, particularly because as I tried different antennas in the same spot the SWR was directly related to the length of the antenna I tried, with the 3ft one having the worst SWR.

Even though the lower part of the antenna carries a substantial amount of the total energy emitted and is still blocked somewhat by the cab in some directions, it is apparent the long antenna overcomes this to a satisfactory degree.

However, I suspect that testing would still show some degradation of the signal, particularly forward of the truck.

In any case, life is full of compromises, and this seems to be a good one.
I haven't had a chance to fully test out the coverage on the road but we'll see. I need to get the radio wired and mounted first, just wanted to go ahead and get the antenna mounting sorted out because I knew I'd be facing some challenges.
 

slowmover

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Joined
Aug 4, 2020
Messages
188
Location
Fort Worth
I'm trying to install an antenna on my 01 F-250 but can't really get below a 2 for my SWR. I'm using a Firestik bed stake mount in the hole closest to the cab of the truck. The antenna is a 4ft tunable tip Firestik.

Strangely enough, when I measured the SWR after a heavy rain with the mount wet it was down to 1.3-1.5. When I dried the mount off it jumped back up to just over 2. So I poured a little water on the mount again and it went down again. What would cause this? I have continuity between the bed and the mount so I wouldn't think it's a grounding issue but I'm unsure.
Get a Breedlove permanent mount on roof with a fold-over. Antenna not less than 5’.

I run 14’ clearance on the Peterbilt. Used a quick-release set to stow them aside. Might use that yearly, so removed them. This I’ve done for years with trailers needing 13’5” clearance. Adjust your routes.

On the roof of my Dodge, a 7’ PRESIDENT Texas is at about 12.5’. A SIGNAL ENGINEERING Golden Rod 45 is under 11’.

A 108” whip exceeds 14’, but there aren’t many places that’s a problem given some forethought (puck mount with backing plate).

To back down height from 10-11’ is going to entail signal loss.

Height is Might. Worth the trouble, IMO.

A CB (with DSP audio) is more valuable now than ever before.

.
 
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slowmover

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Aug 4, 2020
Messages
188
Location
Fort Worth
That's my theory as well, particularly because as I tried different antennas in the same spot the SWR was directly related to the length of the antenna I tried, with the 3ft one having the worst SWR.


I haven't had a chance to fully test out the coverage on the road but we'll see. I need to get the radio wired and mounted first, just wanted to go ahead and get the antenna mounting sorted out because I knew I'd be facing some challenges.
A pickup with worse ears than a big truck is what you may find you have. Should be embarrassing, as truckers often are looking for information you may have as one who lives locally.

The BIG RADIO is sometimes a mommyvan with a centrally-mounted permanent no-big-deal antenna (like a Wilson 5000) done up by a shop for use as a pilot car for oversized loads. The lady aboard out-talks nearly everyone around her for several miles.

And her radio is probably no more than a typical dual-final export.

Your pickup can have that same quality.
With a roof mount.

The bad choices made by pickup owners for antenna locations guarantees piss-poor performance. Gave away half of what they could have had.

I’ve sunk quite a bit into the Peterbilt installation and gear. It’s almost ALL to overcome the deficient radio rigs I have to communicate with re every sort of road problem on a daily basis.

Big trucks wind up using power as a needed crutch. Everyone — trucker or not — also needs DSP in the RX audio.

If you’ve a base station optimized for local, test your compromised mobile from a 5-7/mile “good” parked location. Then have the driver swap a tri-mag mount to the roof with a 108” tuned previously. That margin can be everything.

Two very bad back-ups eastbound on IH-40 in Tennessee today. Hours to travel 30-miles. In each case one needs reception of more than 3-miles to use the only exit to bypass each.

And enough ummph — fire in the wire — to try to ask locals if the map route is feasible. Big truck or RV. Choices matter.

There are potentially MUCH worse hazards that can occur.

If you missed what the man going the other way tried to tell you, then . . . .

.
 
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