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Mobile Antenna SWR Tests with the MXT275


jas
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Hi All,

 

As a followup to the tests I wrote about  in an earlier post, today I hooked up my Surecomm SW-102 to the Midland MXT275 and ran tests with both of the antennas. I didn’t expect the SWR variation I got using different frequencies. Maybe someone more knowledgeable than I can forward a possible explanation….

 

All the test were performed in an open area, using the Midland MXTA12 Antenna Mag Mount with the original cable length.  The antenna is mounted behind the moon roof in the center of  the rear portion of the roof with steel dimensions of 14 x 40 inches. The rear tailgate/spoiler roof is behind that steel roof, and is about the same dimensions. That portion of the roof is non magnetic (some sort of plastic), and houses all the vehicle antennas (GPS, Wireless, AM/FM, Cellular) with a flat aerodynamic, raised hump in the center. The distance from the hump to the antenna mount is about 10 inches. All those system radios were operating at the time of this test, except for Cellular.

 

I still have not received the 1/4 wave Laird whip I ordered. I will test that antenna as soon as I receive it. Should have it in about ten days as it seems to be back ordered.

 

The results:

 

Midland MXTA25 – “3 DB” Ghost:

 

Channel   1 - 462.5625 – SWR 1.08

Channel 16 - 462.575 – SWR 1.14

Channel 22 - 462.725 ­– SWR 1.15

Channel 15RP – 467.5500 – SWR 1.72

Channel 18RP – 467.6250 – SWR 1.74

Channel 22RP – 467.7250 – SWR 1.74

 

Midland MXTA26 – “6 DB” 32” Whip

 

Channel   1 - 462.5625 – SWR 1.11

Channel 16 - 462.575 – SWR 1.24

Channel 22 - 462.725 ­– SWR 1.21

Channel 15RP – 467.5500 – SWR 1.01

Channel 18RP – 467.6250 – SWR 1.01

Channel 22RP – 467.7250 – SWR 1.01

 

All the best,

 

JAS

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SWR will change by channel. The Phantom antennas are not much more than a dummy load or rubber duck smashed in a plastic pepper shaker. They are tuned in the center of the bands. Going up or down in frequency will change VSWR. If you were to sweep it you would see a dip at the best frequency. The gain antenna is the same way. the 1/4 wave should be much closer but will still be as others.

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Hello JAS,

 

Kb2ztx is correct, SWR nearly always varies with frequency.

 

The high numbers I see for the repeater channels on the ghost antenna are a bit of a concern in light of the fact that some GMRS radio manufactures, it has been said, may only warrant there radios when exposed to SWR conditions of 1.5:1 or less.

 

The results of your MXTA26 antenna are very good, and they mirror the one I have and use regularly. That antenna performs really well for me.

 

Enjoy.

 

 

Michael

WRHS965

KE8PLM

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Hello JAS,

 

................The high numbers I see for the repeater channels on the ghost antenna are a bit of a concern in light of the fact that some GMRS radio manufactures, it has been said, may only warrant there radios when exposed to SWR conditions of 1.5:1 or less....................

 

 

Michael

WRHS965

KE8PLM

Thanks Michael. Another reason to not use the MTXA25.  When I read what you said about the high numbers on the repeater channels. I went looking on the web. I found an article on the subject of antenna matching, albeit on an Amateur Radio site, where this came up:

 

"Matching a mobile antenna to the requisite 50 ohms is a requirement for several reasons. For example, modern solid state radios are designed to reduce their output power when the input SWR reaches ≈2:1. Some will handle a little more, some a little less. Once matched, the SWR doesn't have to be flat, so anything below 1.6:1 is close enough. Remember too, if the unmatched input impedance of your antenna, is less than 1.6:1 at resonance, you need a better antenna and/or mounting scenario."

 

 

He goes on to mention, and as I infer from your comment  of staying under 1.5:1, that:

 

".......Once the matching is complete, whatever the SWR is (assuming it is under ≈1.6:1) is irrelevant....."

 

I'm now wondering if GMRS radios are also built to throttle power for anything over 2:00:1 and if they do and we don't know about it then there could be a lot of perceived radio issues that have to do only with that throttling.

 

That article is here:

 

https://www.k0bg.com/match.html

 

 

Thanks again,

 

JAS

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I ordered a cross needle Daiwa meter.  Not sure if that was the correct choice but power and reflected power should be a decent relative indicator I'm hoping.  If I'm off base, I'd like to understand why.  I'm a newbie here.  I have a Laird NMO mount, I am running the shortest coax run I could make (about 8 ft of RG58u)  with smooth radius bends and no pinching. I'm running an MXTA26 antenna and an getting seemingly good range and performance.

 

other than antenna grounding and placement, how could I change my installation impedance?  How much difference in impedance is likely by moving the antenna location?  I'm running a kind of lip mount off the hood which places the antenna near the windshield ("A" pillar) similar to many stock automotive am/fm broadcast radio antennas are mounted.  I understand it would be better mid-roof but I was unwilling to mess with airbag and headliner manipulation to allow that installation as well as not wanting the antenna broken while pushing under brush and branches with the 4wd pickup.

 

Am I wrong in  thinking a simple indication of forward and reflected power should be a good way to check the antenna resonance and later compare different antennas?

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Am I wrong in  thinking a simple indication of forward and reflected power should be a good way to check the antenna resonance and later compare different antennas?

SWR 1:1 does not necessarily means that your antenna is at resonance. It means that it's well matched. Assuming there is no big loss in your feed line (that eats up forward and reflected power), the lower SWR, or reflected power, the better the match. And antenna does not have to be resonant to be an efficient radiator. Resonant quarter-wave above ideal ground has 36 Ohm impedance and SWR 1.5:1 when connected to 50 Ohm feedline. So, tune for lowest SWR, i.e. for lowest reflected power. Use good practices to ensure minimal possible loss in the feed line.

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SWR 1:1 does not necessarily means that your antenna is at resonance. It means that it's well matched. Assuming there is no big loss in your feed line (that eats up forward and reflected power), the lower SWR, or reflected power, the better the match. And antenna does not have to be resonant to be an efficient radiator. Resonant quarter-wave above ideal ground has 36 Ohm impedance and SWR 1.5:1 when connected to 50 Ohm feedline. So, tune for lowest SWR, i.e. for lowest reflected power. Use good practices to ensure minimal possible loss in the feed line.

Alex,

I very much appreciate your reply!  It is quite thought provoking.  As an uneducated new guy, I was picturing a resonant whip with standing waves.  The fact is, I really don't know what I even have, the Midland antenna being described as a "5/8 wave, coliniar antenna" and I don't know it is impedance matched by some coil or capacitance in the base or if the "colinear" bit is from the coils near the mid point.  I have much to learn.  It's something I'm not good at but am interested in.  I guess you are saying I am on the right track by adjusting to a minimum reflected power assuming I have no control of other factors besides "good practices".  I was thinking I would only "tune" by sliding the antenna wire in the spring mount.  A motivator for the purchase is to check a friends 275 installation which isn't apparently working as well as mine.  I see now this may not be so meaningful of a data point.  If I learned something, that likely worth it right there.  Thank you!  It is very hard teaching this "old dog" any new tricks.  Won't stop me from trying tho...

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With 5/8 wave it should be a coil that does matching, to balance out the big -jX component, but I may be wrong here - working from memory, and too lazy to check on internets. "Colinear" usually means 2 or more sections in line, but also could mean that Midland marketing dept found a cool scientific-sounding buzzword.

You are absolutely right about adjusting to minimum reflected power. I think moving antenna back and forth on a lip mount will not make a big difference, if any, but worth tying. Good practices means that your soldering and crimping is good and no sharp bends of the cable. I tend not to use RG58U in my car installs, I prefer RG8X or LMR-240. However, if you already have cable and connectors, it's fine. On 8' length difference in attenuation is negligible.

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Hi all,

 

This is an addendum to my original post above. I received the Laird 1/4 wave 6" whip antenna and have now done the SWR test. It's really good. Especially when compared the the Midland MXTA25 Ghost, the results of which you can refer to on the first post. Reception is also crystal clear. Very pleased. with the Laird product.

 

Here are the numbers:

 

Laird  – “2.1 DBi” 6” Whip

 

Channel   1 - 462.5625 – SWR 1.04

Channel 16 - 462.575 – SWR 1.06

Channel 22 - 462.725 ­– SWR 1.07

Channel 15RP – 467.5500 – SWR 1.01

Channel 18RP – 467.6250 – SWR 1.01

Channel 22RP – 467.7250 – SWR 1.01

 

The next step is to install a new Laird Mag mount. It has the RG8X cable vs the RG58/A/U on the Midland mount.  The Midland mount/cable combo has a calculated loss of about 45% so my MXT275 has turned into an ~8.85 watt radio! The Laird low loss mount/cable should up the output of the radio to about 12.3 watts.  It should be here next Tuesday and I'll report back after the install.

 

All the Best,

 

JAS

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Hi all,

 

This is an addendum to my original post above. I received the Laird 1/4 wave 6" whip antenna and have now done the SWR test. It's really good. Especially when compared the the Midland MXTA25 Ghost, the results of which you can refer to on the first post. Reception is also crystal clear. Very pleased. with the Laird product.

 

Here are the numbers:

 

Laird  – “2.1 DBi” 6” Whip

 

Channel   1 - 462.5625 – SWR 1.04

Channel 16 - 462.575 – SWR 1.06

Channel 22 - 462.725 ­– SWR 1.07

Channel 15RP – 467.5500 – SWR 1.01

Channel 18RP – 467.6250 – SWR 1.01

Channel 22RP – 467.7250 – SWR 1.01

 

The next step is to install a new Laird Mag mount. It has the RG8X cable vs the RG58/A/U on the Midland mount.  The Midland mount/cable combo has a calculated loss of about 45% so my MXT275 has turned into an ~8.85 watt radio! The Laird low loss mount/cable should up the output of the radio to about 12.3 watts.  It should be here next Tuesday and I'll report back after the install.

 

You can find my original post about the losses using the Midland Mag Mount in the link below. It's in the pinned Cable types and losses post at the top of this Forum;

 

https://forums.mygmrs.com/topic/252-cable-types-and-losses/?p=23127

 

 

All the Best,

 

JAS

 

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