mobile install options/observations

So a while back I installed an FT7900 and Larsen NMO 2/70, 5/8 wave (great mobile reputation) in my 80 series with an NMO mount through the roof (centered). I have since sold my FT7900 and about a year ago installed a Kenwood TM-D710. Here are a few head scratchers.

Mount options.

Last week I bought a RTT that will now cover the roof so I need an option for when it is mounted and I don't want to drill a second hole in the roof. I was thinking of a hood mount vs. lift gate mount. Anyone have experience comparing TX/RX at these two locations? I want to minimize my performance loss so I want a 5/8 wave.

Here is another question that is a bit tougher. I have an NMO to SO239 adapter that I use for my ATAS 120 and FT857 (stationary). I was wondering if I mount the Larsen, say on a load bar (not grounded) in front of the RTT and plug the antenna into that NMO/SO239 adapter if that would work? Seems the roof would still be the ground plane, with the hood acting as a less efficient ground plane in the forward direction. yes/no?


Here is an interesting observation.

The person that sold the FT7900 to mounted it in a 100 series with a Diamond NR73BNMO 1/2 wave antenna on the rear lift gate, maybe a foot below roof line, right/passenger side. On a trail run a few months ago he was able to hear one of our club members that was just on the other side of a slight ridge and I was not. We did have a rock outcrop to our right and the TX was from left. He was right behind me. I attributed it to possible odd signal bouncing but didn't know if anyone might know off hand of RX sensitivity differences between the two radios? By antenna and mount locations I should be out performing him. Other explanations?

Any thoughts on any of this?

Two somewhat pedestrian comments- I have tested a few radios and discovered that they are different, even when they are supposed to be the same. Like my vx-3r on 1watt won't hit the 145.310 repeater from my driveway, while my vx-7r On 1 watt will hit it. Both using the exact same antenna. So it's definitely possible that the Yaesu just receives better than your Kenwood. I trust you were turning your squelch down?

Second is I have resolved to mount my antennas more for antenna preservation than optimum propagation. Lots of us here with wagons all got our first radios around the same time, all mounted the antennas on the hatches, and most all have broken at least one. Some still fight that battle, others have lowered them to transmit another day. It's only in those circumstances where someone else can hear and you can't that it makes a difference, which for me has been at most about once per year. I have been ok with that compromise...

nakman said:

.. I trust you were turning your squelch down?

Click to expand...

Dang it, didn't think of it at the time, seems like a real possibility. I hate when I overlook the obvious. I will keep that in mind for next time.

nakman said:

Second is I have resolved to mount my antennas more for antenna preservation than optimum propagation. Lots of us here with wagons all got our first radios around the same time, all mounted the antennas on the hatches, and most all have broken at least one. ...

Click to expand...

My buddy has broken two antennas on his hatch mount, that gets expensive.

When I get in low overhang places I just remove my roof mount antenna, cap the NMO connector and put up a small mag mount quarter wave (MFJ). I have an antenna switch under the drivers seat so I don't have to unplug antennas. Takes 1-2 minutes tops to swap. This is mainly for rig -rig comm but the little 1/4 wave does ok reaching out a bit. This way I can't still get the max propagation most of the time. I really don't need the 1/4 all that often as I find I can pick lines well enough to avoid branches. It is generally the sweepers that make me change over.

This is my solution to low hanging stuff, the STI-CO Flexi-whip. At least for 2m, the rear one. The front antenna is a Larsen for 70cm and both are 1/4λ and I run a duplexer. Anyway, the Flexi-whip is extremely durable and flexible, it's claimed it can be tied into a knot. Never tried that but it's lasted several years up there and I never remove it just about ever, including driving into the garage, parking structures, etc. It just flops over and pops right back straight.





If I find I need extra range I have 5/8λ whips that I can put on in place of the shorty whips but I have no trouble making Denver repeaters from up here even at low and mid power (5W and 25W), so I don't find that necessary much. Most of that is due to the ideal ground plane, it really makes a big different over lip mounts even with unity gain whips. I can't stress enough how well drilling an NMO mount in the middle of your roof works... Don't have to deal with large antennas and get essentially zero fade since a 1/4λ has equal gain through complete azimuth and elevation.

Running independant VHF/UHF is the best way, I thought I was brave for drilling one hole

That antenna whip flexibility looks similar to my MFJ mini mag mount though I have not tried to tie a knot with it but for $20 maybe I should. What I really like about that one you have is that it is an NMO. I may look into getting one of those and ditch the mini mag mount except for use in vehicles with no mobile install. Thanks for posting that.

Interesting that we have similar but different approaches here. I keep the 5/8 wave on all the time and switch to the 1/4 wave when needed and you keep the 1/4 wave on and carry a 5/8 wave. I guess the take home message is you can make a NMO roof mount work for a variety of conditions if you take a second antenna and that really is not a burden, IMHO.


Any thoughts on this part of my questions/post, I modified a bit for clarity:

I was wondering if I mount the Larsen, say on a load bar (not grounded) in front of the RTT (with a second NMO mount) and then plug the antenna into that original, centered NMO with SO239 adapter if that would work? Seems the roof would still be the ground plane, with the hood acting as a less efficient ground plane in the forward direction.

Let make sure I understand, you are using a plane-mounted NMO (or mag-mount, lip mount, it doesn't matter really) with a SO239 adapter to a short coax feedline to a bar-mount antenna?

Electrically it will show conductivity but you can't necessarily assume it to be a good or universal ground. Reason for this is the radiating element itself needs to couple to the return, so the distance and coax impact the characteristic of this. The physical distance and material used between the antenna and the mount and around the ground plane aren't random.

Short answer would be that it will work but there is a performance impact. How much? Probably not enough to notice normally but you are losing efficiency, so it will ultimately be non-ideal. The nice thing about the NMO mount is it's a uniform impedance across it, so the discontinuity will be primarily in the SO239 & PL259 parts.

Also, yes, the sheet metal of the truck will still act like a ground plane. The difference is that you have to visualize the extra mount and coax as an impedance between the hot and return. You will be forcing RF current on the shield, which is a restriction but no worse really than any other feedline.

Would a sketch be useful? What will actually happen is the short patch cable will act as a small antenna most likely. I'll have to mull it over, but you might want to put a choke on the outside, although if it's short I don't know that it will matter much either way.

DaveInDenver said:

Let make sure I understand, you are using a plane-mounted NMO (or mag-mount, lip mount, it doesn't matter really) with a SO239 adapter to a short coax feedline to a bar-mount antenna?

Click to expand...

That is correct, about a 3 foot pig tail would take me to a Yakima roof bar that will only be on when I have the RTT. I agree it would be sub-optimal, just how much.

I understand what you are are saying, I think. But as you stated, for a short run, it may not make a difference. I wish I had an antenna analyzer, that could help answer some of these questions. NMO mounts are cheap so maybe I can just try it out. It think a sketch could help the discussion along, as I am not sure I understand the PL259/SO259 discontinuity concern.

I have an MFJ-269. Don't mind helping at all. If it works in your installation, I wouldn't worry much about it. The truth is all the connections are more of a problem mechanically, lots of places to break and leak.

Thanks for the offer to help, too bad I don't get that far north too often. Based on what we discussed I think I am going to order an NMO mount and give the bar mount a shot. I hate to use so many connectors, but in the end it may not be that bad.

I looked up your Flexi-whip and I did not see a dual band version, bummer. I will need to find another one now that I have the idea of a flexible 1/4 wave NMO mount stuck in my head.

why not run a fender mount?

Up above I said hood mount, what I meant was fender mount.

I think up on roof would be better ground plane and I would not have to route another cable and have an exposed mount all the time. This is a secondary antenna location for when I have a RTT mounted.

ahh, got it. Didn't realize this was secondary.

So our family found ourselves in Denver for an unexpected trip today so I stopped in at HRO to pick up some antenna supplies. Got an NMO mount that I soldered up tonight. While looking at small 1/4 wave antnennas the guy said a 2m 1/4 wave would work on 70cm but not perfect. Looks like this antenna would be 3/4 wave on 70cm. For my uses I think that would be aceptable so...I think I will order up that Flexi-whip

Just need to quickly fab up a mounting bracket for the new NMO and try it out. It may be a couple of days before I can get to it.

Dave, If you have an MFJ-269 you should be able to check the SWR across the 70 cm band on the 2m 1/4 wave Flexi-whip. Would that be too much trouble?

Yes, he's right, that a 1/4 at 145MHz is just slightly longer than 3/4 at 450MHz. IIRC it's about an inch different. Sure, I'll measure what that Flexi-whip looks like.

OK, it'll tune but you have to very careful. I started with a whip that was slightly long on 2m and it was about 2:1 on the upper end of 70cm. For reference we get 144-148MHz on 2m and 420-450MHz on 70cm.

Starting point:
144 - 1.3:1
145 - 1.4:1
146 - 1.5:1
147 - 1.7:1
148 - 1.9:1

This gave:
420 - 2.8:1
425 - 2:1
430 - 1.3:1
435 - 1.5:1
440 - 1.6:1
445 - 1.3:1
450 - 1.6:1

I knew this was long and not ideal but didn't realize it was quite that bad. So I trimmed this antenna to minimize at 146MHz, which got it down to 1.2:1 at the middle of 2m. But this made 70cm go completely useless, 3:1 and worse throughout our band. It's perfect for 2m now, though! I actually knew this would be the case since the 2m and 70cm bands do not line-up as perfect multiples, so I figured if/when this happened I would either have a perfect 2m antenna or could always just cut it to a 1/4λ on 70cm.

Non-the-less, when you sweep a 3/4λ antenna it will not respond with a flat SWR curve as the various characteristics dominate. It's kinda like a 1/4λ sometimes but then it'll look a little like a 1/2λ or 5/8λ mixed in sometimes.

So you can't just pick a single frequency to dip and aim for it, you have to sweep across your bandwidth and pick a happy compromise. I let the lower part of 70cm do what it would since here in Colorado and most places the lower frequencies are SSTV, EME, SSB, weak signal and QRP. Since this is on my truck which transmit on FM for simplex and repeater phone only, I tuned from 433 to 450MHz, so let it bounce around 1.6:1 across there.

Trimmed my second:
144 - 1.2:1
148 - 1.6:1

This gave (approximately):
420 - 3.2:1
430 - 2:1
440 - 1.5:1
450 - 1.6:1
460 - 2:1

Awesome data set, thanks!! Based on your retuning to 146 mhz , it appears that on 70cm in the band range I am interested in for phone 445 - 448 the SWR should be just over 1.5, certainly acceptable.

This seems like it may be a good option for me instead of using my 1/4 wave mini mag mount, even if it is quite a bit more expensive.

Does the antenna come with a trim guide for frequency? I think I may be placing an order tomorrow. I found a couple of places that have it for ~$65, where did you buy yours?

I get them at Tessco. Should be about $35 for one without a mount, which is the STI-CO part number ROOF-FT-NITI-M.

They are $60 if you get the ones with a mag mount or that include an NMO mount. What your are looking for is the replacement whip option, no connector, no mount. Well, unless you need a mount or NMO, in which case the replacement whip is not the right option.

As I recall they just come in a plastic bag with a rubber seal and not much else. They are targeted for commercial installations and I suspect they buy them by the case, so they don't need instructions that will just clutter up the shop.

http://www.protelecomsupply.com/sti-co-industries-inc-flexible-whip-only-for-roof-mount-antenna

https://www.tessco.com/products/displayProductInfo.do?sku=446637

Unfortunately it appears no one sells the whip and base, don't need the NMO mount but I may be stuck buying all of it. Tomorrow I will call STI-CO directly.

I found the cut guide on the website.
View attachment ci-304a-ROOF-FT-NITI.pdf

The mast length (includes whip and what they call mast adapter (I would call it base)) for 145 Mhz should be 19.8". Do you recall what you wound up with?

I reread your results above and did so a bit slower and I became a bit confused by your saying when you trimmed/tuned for 146 Mhz the 70 cm went useless at worse than 3:1. But farther down I interpret the trimmed whip for 146 Mhz yielded a SWR of 1.2:1 and the resulting SWR for ~445 Mhz was slightly more than 1.5:1. It seems like SWR for that portion of 70cm actually improved over the original length. Maybe I am missing something Sorry, not trying to be a pest.

Tessco has sold the whip on an NMO base for $34, that's the link I put in there. It does not come with any NMO, but screws onto my Larsen NMO mounts just fine. Be sure to use the thin rubber gasket they include, anything much thicker and the center contact may not reach the NMO base.

When I give a frequency, it's literally that frequency, not a shorthand for the band. IOW, 146MHz is 146MHz, not a substitute meaning the 2m band.

It takes some finesse and will have to be done carefully (I was taking 1/16" off at a time), but it looks like it can be done. It's also important to note that I have not done any testing while driving, this antenna is very flexible and my suspicion is that the SWR will swing fairly wildly as it moves around.

The take-away is you will have the SWR get beyond 2:1 somewhere in either the 2m or 70cm band and if you're not careful it will be in the FM phone portion in one of the bands. I selected the longer of the two values because I use 70cm quite a bit so wanted it to be low SWR on 440MHz through 450MHz. The ideal VHF length for my truck would be somewhere between these two cut lengths, I went too short by ~1/4" with 18-5/8". It would be fine on 2m, however it would be a very poor choice for UHF. It's 2.1:1 right in the center of my intended use on 70cm.

BTW, I will probably not need the shorter one if you're interested in it. How about $20? This would be definitely good for a 2m-only installation if someone else is following this thread.

I'm constructing a mathematical model for this, hopefully I get that working today or tomorrow. When I do I'll know more about how this antenna is operating.

Measured data with these conditions:

• NMO-K on roof, approximately the middle left-to-right.
• The mount is roughly 12" from the back and 36" from the windshield towards the rear of the cab
• Cut to length RG-58 coax, runs under the headliner, down the B-pillar to under the passenger's seat
• MFJ-269 analyzer
• The value is OAL as measured from the tip of the whip to the bottom of the NMO base, the actual whip is about 1/16" shorter
• Line is smoothed with a cubic spline