Need help with 3/8-24 mounting solution

[ClemsonSCJ]

Additional Update:

Made contact with some local guys that are always talking on channel 40 on my way home earlier. Most of them are running base setups and they were probably 5-6 miles and coming in clear as day and said I sounded fantastic. What really impressed me though was another guy that chimed in also running mobile, except he was barefoot. I measured where he said he was at to where I was at when we were talking, on google earth, and it measured dead on 12 miles (linear, not driving). He was barefoot and I could tell cause he was pretty faint on my end even with the pre-amp (which from my experience pre-amps still don’t make up for your transmit distance so there are far more people hearing you than you can hear in return). But he said based on the way I was coming in on his end he would have never known I wasn’t a mile or less down the road.

All in all I’m glad I went with the Larsen antenna, and this President Bill is worth its weight in gold considering it has far more features than your “traditional” compact CB’s (like the Cobra 19 DX IV I just got rid of) and somehow manages to be about 1/3 the size of those same CB’s.

For anyone who manages to come across this thread in the future, here’s my takeaway:
3/8-24 mount antennas should be reserved for the trucking market and the trucking market only. Any regular vehicle needs to stick with a UHF or NMO mounted antenna if you want the best performance coupled with the best aesthetics.

 

[RFI-EMI-GUY]

Just an update. Finally got everything set up and tuned today. The final product wound up being a President Bill to a RM Italy KL-203P to a Larsen NMO27. Had to trim about 1/2” off the antenna and best I could get the SWR was about 1.4 or 1.5 across the entire band. Kinda weird cause my Laird CB27 on my Jeep is almost the identically same antenna and I got 1.4ish on channels 1 and 40 and it goes down to 1.2 or 1.1 around channel 19. Never had one that stayed exactly the same across all channels. I didn’t mount it dead center, but rather more towards the rear, which is exactly the same as on my Jeep, in case he wants to do a dual band in the future.

Are you checking VSWR with the KL-203P in line? If so, check it straight through from radio to antenna. Also the doors need to be closed when checking VSWR as those can affect the numbers. Otherwise, those are still good numbers.

 

[mmckenna]

Glad to hear it worked out well.

I've never regretted doing a permanent NMO install, and I learned to avoid gimmick/hobby/ham grade antennas a long time ago. Never had a good Larsen antenna fail me.

 

[ClemsonSCJ]

Are you checking VSWR with the KL-203P in line? If so, check it straight through from radio to antenna. Also the doors need to be closed when checking VSWR as those can affect the numbers. Otherwise, those are still good numbers.

Yeah that was with the amp taken out of the equation completely and all doors/windows closed and everything. I’m not gonna lose sleep over it by any means, I was more or less perplexed by the fact that there was no “dip” towards the center of the band. It may be a fairly common thing overall, but I personally have never done a radio setup where the SWR wasn’t .2 or .3 lower towards the center of the band once they were equalized on 1 and 40.

 

[ClemsonSCJ]

Glad to hear it worked out well.

I've never regretted doing a permanent NMO install, and I learned to avoid gimmick/hobby/ham grade antennas a long time ago. Never had a good Larsen antenna fail me.

I wish I had a way to do some scientific testing between my Laird CB27 and his Larsen NMO27. Cause on a side by side comparison they look almost identical with just some minor cosmetic differences down around the base of the coil. The only complaint I might could say that I have about the Larsen is that either the cone that holds the whip or the whip itself may have gotten a little over painted cause where the paint starts on the whip it gets super tight trying to push it into the base. So when I try pushing the whip into the base it’s hard to tell if it’s seating all the way. I think tomorrow I’m gonna try and take a drill bit that’s whatever size of that hole and run it down in there with my fingers to knock loose any thickened paint areas or burs that may be in there and also a piece of 200 grit sandpaper to the bottom couple inches of the whip. I’m actually kinda curious if that wasn’t the issue with the SWR being the way that it was.

 

[mmckenna]

Here's a Larsen NMO-27 permanent mount towards the back of the cab of a 2011 F-150:

The antenna isn't super broad band, so you should see some sort of curve to the SWR. But it may not show up depending on your SWR meter.

 

[mmckenna]

I’m gonna try and take a drill bit that’s whatever size of that hole and run it down in there with my fingers to knock loose any thickened paint areas or burs that may be in there and also a piece of 200 grit sandpaper to the bottom couple inches of the whip. I’m actually kinda curious if that wasn’t the issue with the SWR being the way that it was.

That's a good idea. As long as it's making good electrical contact, it shouldn't matter, but it won't hurt to give it a go.

 

[ClemsonSCJ]

Here's a Larsen NMO-27 permanent mount towards the back of the cab of a 2011 F-150:

The antenna isn't super broad band, so you should see some sort of curve to the SWR. But it may not show up depending on your SWR meter.

What I’ve got it on is a 2011 as well, so I’d definitely like to see closer to those numbers. I’m using a Workman HP201S meter, which I thought was some cheap meter but when I thought I had lost it and was looking at getting another they are apparently pretty damn expensive. I will say that it was a rushed tuning and I was also having to run the meter in a cramped space and upside down. Once I get those 2 things cleaned up I’m gonna try and pull some slack out of both lines so I can run it right side up and know I’m setting everything correctly.

 

[mmckenna]

Well, that spectrum analyzer with tracking generator that spits out those graphs is in the $20,000.00 range.

Workman isn't a high tier brand, and it would vary based on how well it's calibrated before using it.

 

[ClemsonSCJ]

Well, that spectrum analyzer with tracking generator that spits out those graphs is in the $20,000.00 range.

Workman isn't a high tier brand, and it would vary based on how well it's calibrated before using it.

Oh I’m certainly not claiming to have any great meter by any means, but it certainly surprised the hell out of me when it said the current retail was around $120 cause I know when I got this thing and I didn’t recall being serious enough about radio at that point in time to have spent $120 on an SWR meter.

 

[mmckenna]

Oh I’m certainly not claiming to have any great meter by any means, but it certainly surprised the hell out of me when it said the current retail was around $120 cause I know when I got this thing and I didn’t recall being serious enough about radio at that point in time to have spent $120 on an SWR meter.

Yeah, no way they are worth $120.00. Used to be MFJ made meters like that, and you could get them in the $40 range, and that was still high priced for what it was. Maybe $15 worth of parts inside.
But it should work well enough for what you are doing. If it's still reading high, try making sure the antenna center tab is making good contact with the NMO mount. Make sure you are well away from everything conductive. Best place to do this testing is out in the middle of a field or parking lot with no light poles nearby. Nearby antennas/metal can reflect.

 

[merlin]

Personally, I would stay with the NMO mount and a base loaded NMO antenna.
The footprint of the base is enough you won't rip the antenna out of the roof. Spring base even less likely.
2 NMO mounts on my Ford sedan, I swap M-125, Motorola VHF and UHF(like the Laird)
I have an AS low band (looks like M-125) with NMO mount.

 

[merlin]

Oh and I ended up just ordering a Larsen NMO27 antenna today. I’ve basically just decided that until President wants to figure out some better way to mount their antennas that isn’t going to limit us to compromised mounting locations, or force us to spend tons more money in specialty mounting systems to make them work, their antennas just aren’t worth fooling with.

Those mounts are for mirror brackets on trucks, Every truck stop has them. Bundled with the radio to make a buck.

 

[ClemsonSCJ]

I pulled the whip out today and sanded down all the paint to bare metal and changed nothing else about it. Put it back in as-is and checked the SWR at the same place as before and it’s now 1.8 on channel 40 and 1.4 on channel 1. So I attempted to shorten it about an 1/8” and that made it 1.9 on 40 and 1.5 on 1. So I went back up past where it originally was and got it down to about 1.7 on 40, 1.3 on 1, and it’s about 1.5-1.6 on 19. So whatever I did by taking the paint off the whip it did not like at all. I pulled the base off and pulled the spring center contact out just a tad to make sure it was making good contact and that didn’t change anything at all.

 

[ClemsonSCJ]

First pic is it’s location on my dad’s truck. Second pic is my Jeep. Pretty much identical locations. Don’t remember what my SWR is but I do remember it was really good. I think 1.3-1.4 was highest and around 1.1 or 1.2 on 19.

 

[mmckenna]

I pulled the base off and pulled the spring center contact out just a tad to make sure it was making good contact and that didn’t change anything at all.

OK, so you tried all the stuff I'd suggest. The fact that the SWR was moving up and down a bit was a good indication that things are working as expected. With the antenna to the rear of the cab like that, slightly high SWR would be expected. Just tune for the lowest SWR you can get on Channel 19 and be done.

 

[ClemsonSCJ]

OK, so you tried all the stuff I'd suggest. The fact that the SWR was moving up and down a bit was a good indication that things are working as expected. With the antenna to the rear of the cab like that, slightly high SWR would be expected. Just tune for the lowest SWR you can get on Channel 19 and be done.

Yeah when I get a little time another day I’m gonna check the SWR on my antenna and then swap his on there and see what I can get with the Larsen. And then possibly do the same on his truck with my antenna. If each vehicle has similar numbers, low or high, with both antennas then I’ll know it’s just the mounting location.