confused about antenna length calculation

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LIScanner101

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I just bought a 1/4 wave NMO mount whip for 800MMhz stuff when I travel to some areas like Portland OR where pretty much everything is on TRS 800MHz. Anyway, I got a Laird rated for 806-866MHz. The specs say the whip is 3" and by golly it measures exactly that. However, according to the calculation I normally do for wavelength (243/MHz = 1/4 wave in feet) and work backwards this 3" whip is really cut for around 920MHz. For, say, 853MHz I calculate 3.41". Why is this Laird cut so short? I know it's only 0.41" we are talking about but at this band every fraction of an inch amounts to a large change in frequency.

Thanks for any assistance.
 

majoco

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If you have exactly the calculated length over a flat ground plane, the impedance will be about 72ohms. They probably made it shorter to bring the impedance down to 50ohms. There could also be a small matching transformer in the base.

But like they say, radio is an exact science +/- 50%! :roll:
 

Halfpint

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But like they say, radio is an exact science +/- 50%! :roll:
Or...{VB GRIN!} as we used to tell the `new hire' in the shop... "It's FM." {SNICKER!} "Despite all that `book learning' you are the `apprentice' and if you listen and learn you *just might* eventually become a full fledged `wizzard'. Here's you broom. Just don't do what Mickey tried to do. You might not be as lucky." (And... I *will* admit that after all these years I still sometimes have to remind myself that at times it is still `FM' the way things change on sometimes an almost daily pace. {WAN GRIN!} [*I* started out when tubes were still pretty much `the king' and transistors were called `3-legged fuses'.])
 

jim202

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I just bought a 1/4 wave NMO mount whip for 800MMhz stuff when I travel to some areas like Portland OR where pretty much everything is on TRS 800MHz. Anyway, I got a Laird rated for 806-866MHz. The specs say the whip is 3" and by golly it measures exactly that. However, according to the calculation I normally do for wavelength (243/MHz = 1/4 wave in feet) and work backwards this 3" whip is really cut for around 920MHz. For, say, 853MHz I calculate 3.41". Why is this Laird cut so short? I know it's only 0.41" we are talking about but at this band every fraction of an inch amounts to a large change in frequency.

Thanks for any assistance.

Don't forget that there is some distance between the top of the NMO mount and the point where the whip is attached. Measure this distance and see if the missing length is not accounted for there in the mounting.
 

prcguy

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A 1/4 wave over a good ground plane will have an impedance somewhat less than 50 ohms, and shortening it will raise the impedance with respect to the starting frequency. Maybe you were thinking of a half wave dipole which is about 72 ohms in free space?

When you get above say 500MHz its difficult to go by calculations because things like the thickness of the whip or the ball on top, or the mount, etc, start becoming a large enough fraction of a wavelength to influence the resonant frequency or tuning.
prcguy

If you have exactly the calculated length over a flat ground plane, the impedance will be about 72ohms. They probably made it shorter to bring the impedance down to 50ohms. There could also be a small matching transformer in the base.

But like they say, radio is an exact science +/- 50%! :roll:
 

LIScanner101

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Wow! Thanks everyone, I had no idea this would generate so much interest…!

First off, I made a mistake on the manufacturer – it’s a Larsen, not a Laird. The model number is NMOSPECC800B. The antenna was shipped to me in a sealed Larsen bag, it said 806-896MHz on it.

I took the antenna apart (it’s just held together by a friction fit) and took a photo. The length of the whip is exactly 3”, as measured from the top of the little “button” that is used to contact the center conductor on the NMO connector to the upper inner portion of the plastic cap. I know exactly where this little cap ends because I have a bag of them I bought from the Antenna Farm. The metal rod is 0.062” S/S. The whip is held in place with a tiny Allen screw in the "button" so it can be fine-tuned, I suppose.





To prcguy’s point, I realize that at these frequencies you have to take into account so many things. Maybe as was suggested the height of the antenna “above deck” must be taken into account – perhaps even making the rod thinner or thicker will have some effect on it? And are these antennas “pre-cut” to the center of the rated band – meaning it should be cut to (806+896)/2 = 851MHz?
 
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nd5y

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On those types of antennas everything above the NMO center contact is the radiating element.

You should have measured from the end of the contact, not the top of it. The contact, whip, and ball on the end are all part of the antenna.

Other types of antennas and mounts can be different.
 
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LIScanner101

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On those types of antennas everything above the NMO center contact is the radiating element.

You should have measured from the end of the contact, not the top of it. The contact, whip, and ball on the end are all part of the antenna.

Other types of antennas and mounts can be different.
I didn't measure to the very tip of the antenna ball because it's made of plastic and I didn't think it would have any influence on tuning, but the bottom part of the antenna is metal so I'll get an exact read on that. Thanks.
 

902

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You will not get any noticeable advantage receiver operation by cutting this antenna to exact resonance.

Also, remember that these antennas are intended to present a good match to the TRANSMITTER, not the receiver. The most common operation would be from 806-821, hence the generally longer antenna. HOWEVER, the antennas are broadband enough to perform adequately on talk-around (and for receive).

I've tried this before and saw no practical advantage. I would leave it be and it will work fine.

Good luck if you do experiment and make sure you take before/after off-the-air RSSI or dBm measurements and do share them!
 

LIScanner101

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Yeah, I tend to overanalyze everything, even things like this that probably don't matter. I'm just going to leave it as it is and use it as is. Thanks.
 
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