Custom Mobile Scanner Antenna Length?

[Tinkertek]

In my truck, I'd like to monitor around 155 MHz and also around 860 MHz. I have an existing NMO base mounted in a stake hole. Don't care to listen to anything VHF low band or UHF. If I were to custom cut a whip without any coils, what would be the optimum length of that whip?

Typically 234 / Freq (in MHz) x 12 would give me the length in inches for a 1/4 wave monopole. However, is there any way to cut the whip to about 18" (to be 1/4 wave resonant on 155 MHz) and some multiple of 1/4 wave on 860 MHz (maybe 1 1/4 wave)? What would that optimum whip length be and how to figure the math out?

Thanks!
Paul

 

[jonwienke]

No. When you go greater than 1 wavelength, the radiation pattern changes to vertical rather than horizontal, and you'll be primarily limited to monitoring radios flying above you.

You'll need to make half of a fan dipole with separate elements for VHF and 800MHz, with each element cut to resonate at the freq you want to monitor.

 

[Blackswan73]

An antenna cut to be resonant on the lowest frequency of interest, will also be somewhat resonant on the odd harmonics of the fundamental frequency. For example, an antenna cut to be resonant on 155 MHz., will also be somewhat resonant at 465mhz, and 755mhz. The actual resonant harmonics will not be exact multiples because of other factors, but somewhere around there. This is why a 2m antenna will also work on 440mhz., which is roughly the third harmonics. This only works on odd harmonics, and only when the harmonics are higher than the base frequency. This is also the basic theory of how a discone works, as well as the 2m/440 rubber ducks as scanner antennas.

 

[Tinkertek]

Thanks folks - I've been wondering about how that worked for a while now!

 

[n5ims]

Since you already have an NMO mount, you probably should consider something designed for the two bands you're interested in (even if it includes one or more you don't care about). Something like the Larsen (or other quality brand) Tri-band 150/450/800 antenna (https://www.tessco.com/products/displayProductInfo.do?sku=427126 for example). Sure it may initially look expensive, but if you shop around you can often find it for half (or less) the MSRP that many sites advertise. They're not super hard to find used at even a lower cost.

OK, they also work on 450 that you don't care about. So what, do you listen to every station your radio can pick up? I'd bet that would be a no so why would you spend more on a radio that only tuned those stations, but didn't work as well on them as a more common one that allowed you to tune stations you don't listen to.

 

[scover5555]

I had a 5/8 wave VHF-Hi cut for 155 from way back when I was an LEO for a two way radio. I think it's 40 some inches. I recently put a scanner in my vehicle to monitor 42, 155 and 800. Upon install, all I had was this 5/8 handy and figured it would work until I found something else. Well I never have looked for something else as it works great for all.
At home I use a 1/4 wave VHF for 155 which is about 18". The 42 is a little salty because of the short length, which I don't care about at home and the 155 and 800 boom in loud and clear.
I know there are some who will dis me for what I am going to say, but, I have been scanning since the 60's and the only problem I have found is using an antenna that is too short, never too long. Example my 1/4 wave stated above.
Now this advice is for receiving, not transmitting. Length is important when transmitting because you want the radio wave to go out from the antenna, not bounce back down the coax.

 

[Ubbe]

Your asumptions are correct. A 1/4 wave whip have a low impedance at the bottom suitable for a coax and a high impedance at the top suitable to attach to an end feed 1/2 wave. As jonwienke pointed out going over a total of a full wavelenght will probably point most of the directivity into the lenght of the antenna up in the sky. That's how a beaverage antenna works when lying on the ground and the end pointing at the horizon.

I wouldn't worry too much about the 860MHz reception as the shear size of the antenna would probably be as good or better than a short 860MHz 1/4 whip, as some have experianced.

/Ubbe

Typically 234 / Freq (in MHz) x 12 would give me the length in inches for a 1/4 wave monopole. However, is there any way to cut the whip to about 18" (to be 1/4 wave resonant on 155 MHz) and some multiple of 1/4 wave on 860 MHz (maybe 1 1/4 wave)? What would that optimum whip length be and how to figure the math out?

Thanks!
Paul

 

[cvfd625]

5/8 Wave over 5/8 wave UHF. Does great from VHF-Low to 800mhz.

Sent from my SM-G930V using Tapatalk

 

[Ubbe]

That equals to 1 1/4 wave but with 5/8 you'll need matching coils at the bottom and at the middle.
Can you share the measurements for the coils?

/Ubbe