Newb: SWR Question

[Papageoff]

Yeah, it got rusty Because I left the little screw they include for grounding a different type of cable installed. It rusted and then that spread.

I picked up a new mount this afternoon. Will try to install it in the next couple of days.

Remove the ground wire and try it that way and see what happens it may be acting like a counterpoise,and also your L mount looks kind of rusty from the photo.....

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[Papageoff]

I get that I'm losing some because of it being that low, but the loaded portion is just above roofline so it shouldn't be too bad. I don't want to put a hole in the roof, so that's not an option.

That rusty old mount might have excessive resistance and not effectively grounding the RF current to the tool box or even to the fire ring connector. Also, the signal is not going to radiate well in the forward direction with it below the roof of the cab.

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[prcguy]

Your antenna is not shielded very much due to the wavelength at CB and if anything your best performance will be to the front of the vehicle. If you place a VHF or UHF whip right behind the cab near a window pillar it will shield the signal and the vertical pillar can act as a reflector giving you gain to the rear.

At CB frequencies, even with a 2ft whip on your tool box that is below roof line, the wavelength is large enough where the cab is just a little speed bump. The RF wave propagating off your antenna is much bigger than the antenna.

I currently have an 8ft HF antenna mounted to the inside bed wall on the drivers side about a foot behind the cab and 2ft of the antenna is below the bed wall. My feed point and coax connector are a just few inches off the bed. My best signals on all bands including 10m (CB range) are to the front passenger side of the vehicle.

For those who put a CB whip on a trailer hitch mount at the rear of a pickup, the best signal will be to the front of the vehicle. BTW, the trailer hitch is a terrible place to mount an antenna for many reasons.

I get that I'm losing some because of it being that low, but the loaded portion is just above roofline so it shouldn't be too bad. I don't want to put a hole in the roof, so that's not an option.

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[Papageoff]

Your antenna is not shielded very much due to the wavelength at CB and if anything your best performance will be to the front of the vehicle. If you place a VHF or UHF whip right behind the cab near a window pillar it will shield the signal and the vertical pillar can act as a reflector giving you gain to the rear.

At CB frequencies, even with a 2ft whip on your tool box that is below roof line, the wavelength is large enough where the cab is just a little speed bump. The RF wave propagating off your antenna is much bigger than the antenna.

I currently have an 8ft HF antenna mounted to the inside bed wall on the drivers side about a foot behind the cab and 2ft of the antenna is below the bed wall. My feed point and coax connector are a just few inches off the bed. My best signals on all bands including 10m (CB range) are to the front passenger side of the vehicle.

For those who put a CB whip on a trailer hitch mount at the rear of a pickup, the best signal will be to the front of the vehicle. BTW, the trailer hitch is a terrible place to mount an antenna for many reasons.

Neat. My intuition said that the antenna will work the best in the direction of the most metal under it. We kinda confirmed that with my buddy's tacoma. His antenna is in the back left corner of the bed and he clearly cannot reach as far off the back left as he can forward and to the right. It is why I tried to get it as close to the truck's center as possible.

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[WRCM]

Interesting. Well like I mentioned, it's not actually on top of the box. So, if we use the word under somewhat liberally, then the bed floor is "under" the antenna. I have a pic attached showing it. ...and yes I'm changing the mount to get rid of that rust.

If there is a way to get the bed of the truck to be the counterpoise then I think I'd be in high cotton. Honestly, I thought that's what I'd done by by the ground wire to the frame. I mean eventually all of the body is tied together electrically. I figured that was good enough.

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That spring is part of your radiating element, and being below the plane of the toolbox lid it is being partially shielded by it and also capacitively coupling to it. I would move the bracket up so the feedpoint (base of the spring) is on the same plane as the lid, or even slightly above it. If you still have problems, connect a 9 foot piece of insulated wire to the bracket. Tape up the end and put it wherever is convenient. That's better than a choke because it actually gives the RF somewhere to go instead of back down the feedline.

A Firestick works just fine as long as it's installed properly.

 

[Papageoff]

That spring is part of your radiating element, and being below the plane of the toolbox lid it is being partially shielded by it and also capacitively coupling to it. I would move the bracket up so the feedpoint (base of the spring) is on the same plane as the lid, or even slightly above it. If you still have problems, connect a 9 foot piece of insulated wire to the bracket. Tape up the end and put it wherever is convenient. That's better than a choke because it actually gives the RF somewhere to go instead of back down the feedline.

A Firestick works just fine as long as it's installed properly.

Well, the lip of the box precludes that move. I replaced the ground wire that I had going to the frame tonight. I moved up to 10ga and this time I tied it directly from the bracket to the frame. It made no difference... But I didn't really think it would.

Have too many things going on tomorrow, but I may get to try to move it to the bed Sunday. If that doesn't work, I'll try your idea next.

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[FiveFilter]

I plopped a Wilson 1000 magnet mount antenna on the top of my F250 and get very favorable SWR values and great communications in all directions.

Since I didn't cut a hole in the roof, I brought the coax into the cabin by first laying it backwards on the roof and dropping it between the cab and the bed, and from there pulled it beneath the truck, finding things to run it over to provide support while avoiding the muffler and any moving parts, then inserted the coax connection through a factory hole in the floor under the driver's seat. The hole was plugged from the factory with a grommet, and I cut the grommet plug about half-way through in such a way that the coax could fit in the grommet when it was placed back into the hole.

Works for me. It is easily removed if desired.

Of course, a magnet mount will not work with the late aluminum Ford bodies.

 

[Papageoff]

Got a chance to play with it a little tonight, but I fear I may have done more harm than good.

I clipped the ground wire from the frame ground and reterminiated on a bolt that mounts the bed. The ring terminal I had wasn't perfect and I think some insulation may be caught under the washer. Unfortunately, the washer appears to be perminantly affixed to the bolt, so I had to make do.

Before the change, I was getting readings like I'd noted before, CH1-1.5ish and CH40-1.4ish. After I got CH1-1.2 Ch40-1.5. Then I put the external meter in line and things started getting weird. It jumped to over 4 on both channels. I took the external meter out and tried again, and the numbers started jumping around a lot, but finally settled on similar to what they were before though much less stable. This is what made me think the insulation may be a factor making that ground inconsistent.

I'm going to see if I can find a 5/16" ring terminal for 10Ga wire in town. If I can find one, I'll take the insulation off and just use heatshrink after I crimp it. If I cannot find one, I may just run a stainless self-tapping screw and a star washer into a spot under the bed somewhere. I really don't want to do that, though.

 

[Papageoff]

So, I was thinking about this more this morning. Is there a problem with my mount or maybe the cable that is causing this? I mean, why is the cable shield carrying RF in the first place? I already replaced the mount; should I replace the cable?

I get that a balun will probably be required to fix it, by why is it happening in the first place? Also, why would a dead short to the body/frame not fix it? Surely, that's a lower impedance path than the coax shield.

 

[RFI-EMI-GUY]

So, I was thinking about this more this morning. Is there a problem with my mount or maybe the cable that is causing this? I mean, why is the cable shield carrying RF in the first place? I already replaced the mount; should I replace the cable?

I get that a balun will probably be required to fix it, by why is it happening in the first place? Also, why would a dead short to the body/frame not fix it? Surely, that's a lower impedance path than the coax shield.

You have current flowing on the coax cable if the antenna has a poor ground plane. Being mounted on a box in the bed is not the best groundplane. That current is wasted energy.

Did you buy this antenna new? If not has someone removed some turns from the end of the whip to tune it?

RF current like any electrical current follows the path of least resistance/impedance. Because you have an insufficient ground plane for return current to follow the desired path into the shield of the coax, the current is following the outside of the coax and thus will have a lot of variability. Putting ferrite "baluns" will sop up that wasted current and turn it to heat. But that signal could be better put to use being radiated from the whip.

After you replaced the rusty mount did you check the resistance to the toolbox with an ohmmeter? Is the tool box bonded to the bed in more than two places? The more bonding the better. Check with an ohmeter. Is the bed grounded to the frame and the passenger box? Check with an ohmeter. You want zero ohms. You should be bonding with a large conductor 12 guage or better, the shorter the better.

Is the lid of the toolbox grounded? It could be floating at RF impedance even if it shows DC continuity. Install some metal finger stock (*) at the corners so that when closed it is grounded.

Just something to consider. If improving the ground changes the VSWR for the worse, it may indicate that the "Q" of the antenna has improved, the antenna is more resonant and bandwidth has narrowed and thus it may have to be re tuned.

If lower VSWR favors the lower channels, the antenna might be too long. If it favors the higher channels, it may indicate problems with the ground plane or the antenna is too short.

I am surprised no one has provided this link to the Fire Stick antenna tuning procedure. This should be read carefully and be sure to follow manufacturers recommendations.

Setting the SWR of Your Antenna© - Firestik® Antenna Company

Personally, I don't have the patience required to get these antennas to behave. I would be cutting a hole in the roof and installing a Larsen NMO27 by now. You still need to tune the Larsen, but the variability will be reduced.

(Flame suit on)

* Finger Stock is a product made specifically for RF grounding of cabinet doors.

 

[cmdrwill]

Personally, I don't have the patience required to get these antennas to behave. I would be cutting a hole in the roof and installing a Larsen NMO27 by now. You still need to tune the Larsen, but the variability will be reduced.

EXACTLY what I would do.

Even the Larsen NMO27 on a mag mount is better than what the OP has.

 

[prcguy]

I believe the OP has the no ground plane coax assy and that could be causing the odd behavior. I think the no ground mount has the coax shield floating at the mount.

I would also drill a hole in the center of the tool box and mount the antenna there but I see no reason to go with a Larsen at this point. I think a small ball mount with coax grounded at the mount will calm things down and a 4ft Firestick would be on par in performance with a Larsen and the Firestick is already paid for.

 

[Papageoff]

I believe the OP has the no ground plane coax assy and that could be causing the odd behavior. I think the no ground mount has the coax shield floating at the mount.

I would also drill a hole in the center of the tool box and mount the antenna there but I see no reason to go with a Larsen at this point. I think a small ball mount with coax grounded at the mount will calm things down and a 4ft Firestick would be on par in performance with a Larsen and the Firestick is already paid for.

I can't put it in the middle of the lid since it's a single piece lid. Also, I don't think I have the no ground plane cable. This is the one I have...

Firestik Antenna FireStik MU8R18... https://www.amazon.com/dp/B000X3814G?ref=yo_pop_ma_swf


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[prcguy]

I looked up the diagram for that cable and it does ground at the mount. More ground plane under the antenna would be nice and decoupling the coax at the mount with ferrite should also calm it down.

The tool box is acting like its own tiny island of insufficient size and I'm sure the coax braid is hot with some RF since its the major counterpoise component. There are other annoying things you could try like bond the top of the tool box to the top of the bed rail in several spots, especially a few long wires running from the box lid to the rear of the bed rail. The key here would be wires or braid that ran from the top of the lid horizontal to the top of the bed rail and not down to the bed. That would increase the ground plane slightly at the expense of wires in the way of your cargo and possibly affecting resale value.

I can't put it in the middle of the lid since it's a single piece lid. Also, I don't think I have the no ground plane cable. This is the one I have...

Firestik Antenna FireStik MU8R18... https://www.amazon.com/dp/B000X3814G?ref=yo_pop_ma_swf


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[Papageoff]

I just took some ohmmeter readings around the system and may have discovered an issue.

I found >1ohm from tool box lid to the antenna mount
I found 0 ohms from the body of the box to the antenna mount
I found 0 ohms from tool box to rear door latch on the cab

...all that is kind of what I expected and I think it's ok. What I did find odd was that I found some small level of continuity between the antenna spring and mount. It was about 9kohms iirc.

This makes me suspect the cable is damaged. Would that be a reasonable conclusion?

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[RFI-EMI-GUY]

I just took some ohmmeter readings around the system and may have discovered an issue.

I found >1ohm from tool box lid to the antenna mount
I found 0 ohms from the body of the box to the antenna mount
I found 0 ohms from tool box to rear door latch on the cab

...all that is kind of what I expected and I think it's ok. What I did find odd was that I found some small level of continuity between the antenna spring and mount. It was about 9kohms iirc.

This makes me suspect the cable is damaged. Would that be a reasonable conclusion?

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"I found >1ohm from tool box lid to the antenna mount".

Greater than 1 ohm indicates the lid is probably floating at RF .

Consider my recommendation for finger stock inside the lid cover to short the lid to the box.

"What I did find odd was that I found some small level of continuity between the antenna spring and mount. It was about 9kohms iirc. "

If the radio is connected to the coax, you might be seeing a static bleed resistor inside the radio. Otherwise there could be conductive debris on the mount that is leaking current. 9K ohms is not going to cause high VSWR. But if it is water in the coax, it could indeed.

Disconnect the radio and make sure 1) 9 K leakage is gone. 2) continuity of the center conductor from PL259 center pin to insulated antenna base , less than 2 ohms. 3) continuity of outer conductor from PL259 shield to antenna mount. Less than 1 ohm.

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[Papageoff]

The radio was connected at the time... I didn't think about that.

As for the fingers, could I just use another wire since I have it on hand? If so, should I tie it to the line I have between the antenna base->trk bed?

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[RFI-EMI-GUY]

The radio was connected at the time... I didn't think about that.

As for the fingers, could I just use another wire since I have it on hand? If so, should I tie it to the line I have between the antenna base->trk bed?

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If you use wires instead of finger stock, the impedance will be a bit higher as they will have to be long enough to allow the hinge to open.

Bond the lid to the box, the mount to the box (using stainless screws and star washers), the box to the bed all using the shortest wires possible and stainless fasteners and star washers. You don't want this stuff oxidizing in 2 months.

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[Papageoff]

If you use wires instead of finger stock, the impedance will be a bit higher as they will have to be long enough to allow the hinge to open.

Bond the lid to the box, the mount to the box (using stainless screws and star washers), the box to the bed all using the shortest wires possible and stainless fasteners and star washers. You don't want this stuff oxidizing in 2 months.

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Okie dokie. I may not get to this for a bit. I have some other work to do on the truck this weekend and gotta start getting ready for the trip to my sister's for Christmas.

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