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433 specific antenna working perfectly on MURS ! ? Huh ?

Antenna band specific

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#1 JCase

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Posted 27 October 2020 - 10:27 PM

Greetings everyone,

Ok, I don’t claim to understand everything associated with RF. Heck, there’s a lot of it that’s just plain over my head (pun intended).
But this particular surprise has me totally baffled. I was several new dual commercial band antennas that I just received in case any tuning was needed and since my mind was set to “dual band” mode I grabbed this inexpensive, lightweight, low power antenna and connected it to one of the Dakota Alert MURS 2 watt base units and decided I should check it also. I don’t mind telling you I couldn’t have been happier with my SWR reading, 1.0-1.1 right out of the package. I had a HUGE smile and thought perfectly, finally found an antenna that needed no tuning, not even a frogs hair amount needed to be trimmed off. I went back to testing the other dual band commercial antennas with several different mobile radios and it wasn’t until I finished and started putting my meters, adapters and various odds and ends away that I realized what I had done. I had connected a UHF only (433 MHz specific) antenna to a VHF transceiver, not only that but I got a picture perfect SWR & power level reading using it n a MURS channel (154.570).

My radio tech ALWAYS stressed that I should only use band specific antennas for the band they were designed for and by using a VHF on UHF and visa versa, So, why is this UHF antenna working spot on with a VHF frequency ? I have included a pic of the antenna I’m referring to.

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#2 mbrun

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Posted 28 October 2020 - 06:25 AM

That is excellent first-hand discovery on your part indeed. What you’ve discovered is that design of an antenna can make it work well on multiple frequency ranges and this is very, very common.

Consider the worst antenna of them all, the rubber duck. The ones used with an amateur portable radio for example. These antennas allow the radio to service the VHF and UHF frequencies nearing 145MHz and 445MHz. Coincidentally, these are separated by approximating 3:1 ratio.

You most certainly have heard the term ‘wavelength’. One man’s 1/4 wavelength antenna is another man’s 1/2 wavelength or 5/8 wave antenna. Same length of radiator, serving different frequencies. While one length may not work as well on one frequency as it does on another it does indeed work.

MURS and GMRS frequencies are separated by an approx 3:1 ratio also. So don’t be surprised to find a GMRS antenna that works well on MURS too.

So how do you know? If the manufacturer does not tell you, you can test it yourself.

Recently I was watching some YouTube videos of folks using their antenna analyzers to scan the response of their antennas to find all the frequencies ranges the antenna served well. The recent popularity of the NanoVNA makes it possible for the technically inclined to scan their antenna and discover where it performs best.

Again, Nice Find.

Michael
WRHS965
KE8PLM







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Michael

WRHS965

KE8PLM


#3 JCase

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Posted 28 October 2020 - 09:00 AM

mbrun / Michael,

Thank you for the reply and the valuable information. My tech friend never explained this so discovering, even more so accidentally, that this is the case (no pun intended) is pretty exciting. I’ve been involved with radios since I was in my early teens but most of my experience has just been on the average “user” end or level. In the fire service and 911 communications we tend to utilize the equipment as tools of the trade and if there was any kind of problem we reported it to the techs and they worked their magic and eliminated the problem. I always tried to stay clear and let them do their work and honestly didn’t see any reason to learn all of the components of how or why since I wouldn’t be authorized to “fix” the problem, only notify the appropriate department to arrange for them to handle. Since retiring and joining a volunteer, non profit organization and since losing my good friend who was also my tech, I’ve been on a mission to learn more about the “how and why” so that I am able to hopefully solve the problems that occur from time to time. If I can save myself or the nonprofit from incurring an added expense then it’s a win win.
Again I thank you for taking the time to reply and providing an explanation that even I am able to understand lol.

Best regards,
John

#4 AdmiralCochrane

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Posted 28 October 2020 - 08:56 PM

I picked up an antenna from a pile of demo stuff not knowing what it was. It turned out to be labeled as a 2gig unit.  It was enclosed in a plastic case so I cracked it open to measure the actual element to see if it might have some use for me on VHF or UHF ... the more I thought about it, the more I decided the only way to tell would be to hook it up with the swr meter key the mic and find out.  I might have to add or subtract one element or it might be completely useless, but only the swr meter will tell in the end.  







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