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  #11  
Old 02-17-2014, 10:57 AM
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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.
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  #12  
Old 02-17-2014, 02:41 PM
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ahh, got it. Didn't realize this was secondary.
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  #13  
Old 02-17-2014, 09:47 PM
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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.
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Last edited by CO Hunter; 02-17-2014 at 11:24 PM.
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  #14  
Old 02-17-2014, 11:23 PM
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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?
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  #15  
Old 02-18-2014, 06:39 AM
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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.
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  #16  
Old 02-18-2014, 02:36 PM
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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
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Last edited by DaveInDenver; 02-18-2014 at 03:05 PM.
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  #17  
Old 02-18-2014, 10:39 PM
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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?
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  #18  
Old 02-19-2014, 06:55 AM
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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-...-mount-antenna

https://www.tessco.com/products/disp....do?sku=446637
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  #19  
Old 02-19-2014, 09:39 PM
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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.
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.
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  #20  
Old 02-20-2014, 01:05 PM
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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

Name:  combined_measured.jpg
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Last edited by DaveInDenver; 02-20-2014 at 01:36 PM.
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