NMO center tab size question

[syntor9k]

Hi,

I have a NMO center tab size question (an antenna problem). I've heard over the years, that some size MNO center tab and bands don't work well together. Is this true? Is there a white paper or some sort of documentation out there (link please). I've google the subject and not found anything.

I have a full size truck I'm working on. It has 3 antennas across the roof. 11m, 2m/70cm (1/4w trap shorty), and 6m. The 6m antenna SWR is around 1.8 at residence and into 2.5-3.5 SWR at the band edges (5 MHz spread). I have removed the 10m antenna and this slightly makes the SWR better on 6m, but not by a lot. The duel band antenna is around 20 inches tall and has a coil in the center (so i would not expect this to be effecting the 6m antenna. Unlike a 5/8th wave on 2m would do).

The NMO kit used through out this install, was the small tab type, with low loss cable.
I have put a NMO to UHF adapter with a load on the offending mount and the SWR is flat. So I think the coax and mount should be OK.

I had an antenna that worked well in that location, at one time, but it got hit by a tree. I found an old AS antenna that was cracked but worked, that we used till the new antenna came. The AS antenna has lower SWR. Thinking that the new antenna was bad, I replace it (that one was around 2.0 SWR). I've tried several other antennas there, with the similar results.

My question:
Is there list somewhere, suggesting NMO center tab, to preferred band usage, for the antenna used?
Or do you have any suggestions for my antenna problem...

[RadioSouth]

I recall with OEM Motorola mounts there was a removable washer that was supposed to be removed when used at the lowest of the spec'd. VHF splits which was the Air band, perhaps this would apply to the 6m, 11m antenna's as well ? Sounds like they should be checked one at a time to rule out interaction.

[d119]

I've also heard some noise that there are differences between the mount "center pin" diameter. What I was told is that the larger tabs are for things like low-band, whereas the "normal" ones are for VHF and above.

I've also heard the large tab was needed for 1200MHz operation, but the friend that told me about that has since passed away.

I would NOT take that as gospel, and I also would like to see some documentation/data explaining the differences.

[RadioSouth]

Oops I had that backward, just found the sheet that comes with Motorola NMO mounts. The models listed cover from 136-870 in 6 splits and it mentions that the spring washer and dielectric washer are to be used only for 136-150.8 MHz for impedance matching and as d119 mentions this washer effectively makes that center button larger with this washer in place. (Being the sheet covers only 136-870 perhaps that washer would also be needed for lowband).
For my cars I've switched over to the the Larsen NMOKHFUD which is spec'd, 0-6 GHz which has dual shield cable and a full shielded NMO mount, seems to make sense with mount and cable running within a couple feet of side airbags.

[d119]

Weird...

[Birken Vogt]

NMOKHFUD is the one to use. Larsen has a lot of options to choose from but this seems the best. Fully shielded from radio connector to roof mount. No RF leaks into the car.

The NMOKUD used to be our standard but it has the short little exposed section on the underside that leaks a decent amount of RF. I have compared the two and it makes a big difference with in-vehicle cell boosters like you'd find in an ambulance. Probably makes no diff at low band or high band.

The NMOKUD (non-HF) does have a much better "bite" into the underside of the roof to keep the mount from spinning when tightening the nut. I usually do it blind from the top so this can be quite annoying.

They make two dual shield cables, DS and UD. DS is harder to work with because it has a stranded center conductor, harder to push into a PL-259. UD has a solid center which is much nicer and also I believe makes the specified frequency it is allowed to use a bit higher I assume because of the more uniform distance from the smooth center to the outside shield.

[syntor9k]

Guess what I have already sitting on my desk for my own install NMOKHFUD.

I plan to try another tow antennas on her roof. But a new NMO mount might be in order. If the next antenna works (a Child's antenna, 5/8th duel band (and yes, it seems to work on 6m quite fine (more testing needed)). If not, I'll use it.

My last try, was a Larsen wideband (40-50MHz). Apparently it does not take tuning by cutting very well? Otherwise the antenna worked fine on my car? Although it is quite wide at 9MHz+. Its now 54in and it was 64in. I was trying to move it to 45-53 MHz. I got it to 52 MHz so far.

Sorry I missed these posts. Its strange, I didn't get message that my post went through?

[Birken Vogt]

A couple of times we made our own "wideband low band" antenna.

Spring base, I think the part number is Larsen AS or something like that, drilled out to accept a larger diameter 62" (IIRC) CB whip which is also larger in diameter. So there was no base loading coil. Worked good electrically. Whether that base was strong enough for the years I don't know.

[Andy Corbin]

FWIW, I seem to remember a Larsen NMO-150 cut to 146 was resonant of 6 meters. Can't recall whether I actually did that or was told. Been awhile since I have dealt with that breed of larsen.

Andy

[syntor9k]

Its I think common that ~150 MHz, 5/8th antennas work on 6m as a 1/4-wave.

I did some testing tonight. I dont have time to write it up now, but I compared several antennas, commercial and some made by me (NMO to 1/4 wave whip with a spring). No field strength test, just SWR. The misquotes were real bad. I decided to do all my work from within the car....

[syntor9k]

Here is my list:

Residence (MHz), 2.0 SWR BW (MHz), length (Inches total), and type and brand
-------------------------------------------------------------------------------------------------------------------
46.0, 42.0-48.0, 55.5, NMO, Larsen wide bandwidth 40-50 MHz Antenna. Modified, Cut from 62in to 54in (had minimal effect) Added spring (cutting made SWR worse, but moved residence frequency a little). This Antenna may be better, before cutting (I need a new spring and cant get one)
47.5, 45.5-53.5, 60.5, NMO adapter to 3/8th-24 (No spring)
48.0, 46.0-53.0, 58.0, NMO, Child's 5/8th wave 2m/70cm (6m 1/4 wave)
49.0, 43.0-54.0, 57.0, NMO to 5/16-24 All-Black (with spring) from Larsen parts
52.0, 48.0-56.0, 55.0, NMO to 5/16-24 Chrome (with spring) from A/S used parts

[TWEMARS]

Oops I had that backward, just found the sheet that comes with Motorola NMO mounts. The models listed cover from 136-870 in 6 splits and it mentions that the spring washer and dielectric washer are to be used only for 136-150.8 MHz for impedance matching and as d119 mentions this washer effectively makes that center button larger with this washer in place. (Being the sheet covers only 136-870 perhaps that washer would also be needed for lowband).
For my cars I've switched over to the the Larsen NMOKHFUD which is spec'd, 0-6 GHz which has dual shield cable and a full shielded NMO mount, seems to make sense with mount and cable running within a couple feet of side airbags.

The larger center PIN in the NMO mount is for 136-512 MHz antennas. The smaller center PIN is for 403-930 MHz. The washer/insulator is for below 150.8 only. This is for stock rods and has no relevance with screw on coils i.e. Low band, VHF/UHF gain, etc. EXCEPT the 800/900 gain antennas marketed by Motorola utilize the smaller center pin.

[syntor9k]

Hi all;

I installed the large tab NMOKHFUD coax kit, in place of the one with the smaller center pin. WOW - What a difference!!! Problem solved.

Now I have 1.3 SWR (~48 MHz) at residence. And after pruning, I have now 1.4 SWR on 47.xx MHz, and 1.6 SWR on 52.525 MHz, the highest likely to transmit on frequency. I gave up once it was ~1.6 SWR or lower which it was...

Coax used (approx. Not FME type):
https://www.ebay.com/itm/Larsen-PULSE-N ... 2207779752

Oddly the CB antenna (using the same commercial 27-30MHz coil), is ~1.2 SWR on Ch.20 (and 1.4 and 1.8 SWR at Ch.1 and 40) - is just fine using the small tab coax???

[Jim202]

I have always used the standard NMO mount from Larsen, along with the NMO-Q coil base. This base has no coil in it. Then used I think, the 59 inch long whip. I may have the length wrong, but it is the longest rod they sell.

Mount the standard base on the roof, put the NMO-Q base on the mount and stuff the whip into the base. In most cases, I didn't have to cut the rod at all. Just install it and go. Have used this combo since about 1979 for 6 meter FM use. All my radios are 100 watts and I get great range out of the installation.

In the mid 90's, I would be crossing the big lake north of New Orleans and be able to talk through 6 meter FM repeaters in Canada. Did this just about every day when the band was open. That was just about every day during the late summer into the fall. When weather fronts were moving through the middle of the country, you could almost predict where the propagation would allow for contacts.

Now with the noise floor coming up so high at many tower locations, 6 meter activity has died like a brick trying to float on water. Now the closest 6 meter repeater to me is in Mobile, AL. Not what I would call a usable repeater to where I live.

Jim

[syntor9k]

I inspected the small tab NMO coax kit. I did-not find any issues. The Larsen large tab NMOKHFUD did the trick?

Anyway, I agree the NMOWBQB and a long black whip 56+ inch whip is a good antenna. The problem is finding the 56-62in whips, especially the black ones...

Recently, I found a new antenna that I like, it covers 6m (unofficially) and is a single 5/8th dual band (146/446 MHz) antenna, made by Child's:
https://www.ebay.com/itm/VHF-UHF-MOBILE ... 2172262970
The bad news its not black (I painted mine).

[syntor9k]

OBTW. And yes the noise floor issue is real bad in many places...

[motorola_otaku]

We use the NMOKHFUD kits at work, and I always had misgivings about how much power that tiny center pin underneath the button could handle. Seems like there'd be more than a little loss at 110 watts.