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Quarter wave versus 5/8th's wave plots

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mmckenna

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I had my analyzer at home with me. Since it's a nice day, I decided to play around with some various antennas, save the plots and do some comparisons. Since pictures are worth a thousand words, here they are:

This is a Larsen NMOQ cut for near the center of the 2 meter amateur radio band. Mount is a permanent NMO mount in the dead center on the roof of a Ford F150 crew cab.
Notice the wide bandwidth. This is where quarter wave antennas really shine. For interoperability uses, it's hard to beat. It's easy to get 2.0:1 SWR across a big chunk of spectrum.
Marker 1 is at the lower 2.0:1 SWR point. at 138.478MHz
Marker 2 is near the center of the 2 meter amateur band.
Marker 3 is at the upper 2.0:1 SWR point, 170.695MHz.
32MHz of useable bandwidth.




This is a Larsen NMO-150 5/8th's wave VHF antenna. Tuned for near the center of the amateur radio band. Same mount as above.
Compare the much narrower bandwidth. The 3dB of additional gain compared to the quarter wave comes at a cost. While 5/8th's antennas are useful for the added gain, they don't cover as much spectrum. Not an issue for someone who is purely using it for amateur radio use, and for a commercial/public safety user that is limited to a narrow band. For interoperability, maybe not the best choice.
Markers at set up like above. Upper and lower 2.0:1 SWR points and the center(ish) of the 2 meter band.
12MHz of useable bandwidth.
 

prcguy

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There is more to it than just the match. When I go from 2m to 162Mhz weather freqs with a 1/4 wave whip tuned near 2m, the local weather channels are full scale on my radio S meter and full quieting. Parked in the same spot and changing to a Larson 5/8 cut for 2m, the weather channels are barely blinking any bars on my S meter and they are very noisy. Some day I'll repeat the test with a spectrum analyzer but there seems to be more than a 10dB difference in reception.
prcguy
 

kayn1n32008

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I had my analyzer at home with me. Since it's a nice day, I decided to play around with some various antennas, save the plots and do some comparisons. Since pictures are worth a thousand words, here they are:

This is a Larsen NMOQ cut for near the center of the 2 meter amateur radio band. Mount is a permanent NMO mount in the dead center on the roof of a Ford F150 crew cab.
Notice the wide bandwidth. This is where quarter wave antennas really shine.
Plus the high angle of radiation makes the 1/4wave great for mountain use. Honestly, I have not used a 5/8wave since I had a mono-band mobile. I have been extrememly impressed with my NMO2/70, that I do not think the losses from a diplexer would make up for using seperate antennas for VHF(5/8wave) and UHF(5/8 over 5/8wave colinear). Opinion?

What would be the odds of throwing a Larsen NMOWB(1/2wave) on there and plotting it?

My employer has use of frequencies from 140.xxxxMhz all the way up to 173.xxxxMHz, and the NMOWB is the antenna of choice for our radios, cut for 162MHz(middle of 150-174MHz). We also have a splattering of Tram(JUNK), Maxrad(About the same as the Larsen) and Sinclabs(Toughest for sure, but super stiff element) antennas out there as well, it truly just depends on which dealer the radio was from. One favors Tram, another favors Sinclabs, and our local dealer near our head office favors Larsen.

Thank you for taking the time to do these plots (I saw the NMO27 as well)
 
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mmckenna

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Plus the high angle of radiation makes the 1/4wave great for mountain use. Honestly, I have not used a 5/8wave since I had a mono-band mobile. I have been extrememly impressed with my NMO2/70, that I do not think the losses from a diplexer would make up for using seperate antennas for VHF(5/8wave) and UHF(5/8 over 5/8wave colinear). Opinion?
I think you are correct. Many years ago I did check the power output on a 2 meter/70 centimeter diplexer. Losses were not insignificant. I don't have it anymore, or I'd do a test.
Where I'm located in California and where I mostly travel is either in valley's or in the mountains. I use 1/4 wave for everything. As PRC said, rx seems to be better, lower profile, less expensive and works well in the mountains.

What would be the odds of throwing a Larsen NMOWB(1/2wave) on there and plotting it?
You know, I had one right there. I thought about doing it, but then the wife had other plans. Maybe another weekend. The NMOWB I have is waiting for my new work truck, so as soon as that arrives I'll be doing the install. I'll try to remember to shoot it then and post the results. I don't expect it to be a whole lot better, but it'll be interesting to compare the two plots.

My employer has use of frequencies from 140.xxxxMhz all the way up to 173.xxxxMHz, and the NMOWB is the antenna of choice for our radios, cut for 162MHz(middle of 150-174MHz). We also have a splattering of Tram(JUNK), Maxrad(About the same as the Larsen) and Sinclabs(Toughest for sure, but super stiff element) antennas out there as well, it truly just depends on which dealer the radio was from. One favors Tram, another favors Sinclabs, and our local dealer near our head office favors Larsen.
I'm using 144 to about 159 and could probably get away with a 5/8th's, but I don't need the extra height.

I've been using Larsen for almost 30 years now and never had an issue. The NMO-27 I plotted earlier is at least 20 years old and still in good shape. I've been trying a few Laird and Antennex and those have been good antennas, just haven't had them as long. I've never tried the Sinclairs, maybe some day.

Thank you for taking the time to do these plots (I saw the NMO27 as well)
No problem. It was a nice day and I had some free time. I'll try to do some more as I have the opportunity. I'll probably shoot some of the 800MHz antennas I have next time.
 

mrweather

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Excellent work, thanks for sharing!

I've been using a NMO150HW for many years under the assumption it gives me a little more gain than a 1/4 wave and isn't as long as a NMO150. I wonder what plot results the OP would get with that?
 

mmckenna

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Well, not a Larsen, but a Laird:



Trouble with that plot is it's in return loss mode, not SWR. Still, it gives you and idea of the narrower bandwidth. They are wider than 5/8th's wave antenna, but not as much as a quarter wave.
Some will say the 3db gain of the 5/8th's wave is superior, but a 1/2 wave with a proper ground plane will give 2.4dB of gain. That 0.6dB probably wouldn't be noticed by most.
 

mmckenna

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And for kicks, here's a different quarter wave on the same mount used as the above. It's tuned for my work VHF stuff, so it performs best around 159MHz. Still totally acceptable for 2 meters and up to 174.

 
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