Is this REALLY what a mobile UHF/446 antenna looks like??

[Ninja]

Hi all...

I have a set of 3 Yaesu VX177 5 watt UHF handhelds that myself and my friends use when on a UK/Euro road trip for comms between cars.

The 3 sets are programmed with, and operated on the PMR446 channels.

I have found them to be absolutely fantastic pieces of kit, miles better than the 0.5watt PMR446 radios we tried a couple of times, and loads better than CB's. Just stick one in a car, cigarette lighter plug in, speaker mic plugged in and away you go, hassle free comms with a half decent range. Digital squelch prevents static or interference from opening the squelch. Great stuff.

But, I would like to try to extend the range if I can. I find that as with all handhelds, if you are standing out in the open, the range is amazing, over 10-15 miles in the countryside! But... once you get inside the metal and glass box that is a car, the range falls quickly to a mile or two at best, maybe less in a city.

To try to solve this, I bought some of these type antenna from Maplin: http://www.maplin.co.uk/miniscan-mobile ... cification

30cm long and allegedly capable of transmitting on UHF/VHF up to 50 watts. But when I tried them... they were were worse than the Yaesu rubber ruck antenna!

So... I used an online Antenna calculator for 446Mhz and apparently I only need an antenna of 16cm for 446? (1/4 wave)

So I cut down my maplin antenna to just over 16cm as per the below photos.

It looks bloody short to me.... is this correct? I have no SWR meter for UHF and I'm worried about trying it an detonating one of my Yaesu's!

Anyone made a mobile mag mount 446 antenna??

[curny]

That should work a treat!
I used to use a taxi mag mount antenna cut down like this to 16cm
Worked great and swr was fine
(I was running a Kenwood TK862 at 25w)

Curny

[Ninja]

Hi Curny,

Thanks for the reassurance!

Now here's a question... does the correct 16cm whip length include or not include the metal base that the grub screw is in, and the whip inside the long plastic antenna cap?

I know this is splitting hairs but the bottom grub screw section and the top cap add up to over 2cm of length.

Or am I worrying too much?!

[SadToes]

looks right but its not legal. Half a watt with fixed antenna and 6.25 step not yaseu 5 step

[8Bit]

For any frequency, you can calculate the resonant wavelength by dividing 3*10^8 ( or 300000000) by the frequency in Hertz i.e. PMR CH1 would be 446.00625*10^6 (or 446006250 hz).

This will give the resonant height of the wave in meters for a frequency, measured in Hertz.

In this case, a wavelength of 0.672m for CH1. A quarter wavelength would be 0.1681m (or 16.81cm), so a 16cm whip would be approximate to a quarter-wave for the PMR frequencies.

The length of the antenna should be measured from the point where the coax splits within the mount. i.e. the centre core meets the antenna and the outer braid goes to the ground-plane.

This will include the wire under the plastic cap. If the cap were metal, the cap should be included also, as it would act as an extension to the whip.

[Edited to correct spelling and remove some ambiguity]

[8Bit]

looks right but its not legal. Half a watt with fixed antenna and 6.25 step not yaseu 5 step

I remember seeing somewhere, the fixed antenna rule for 46Mhz PMR has been relaxed. No?

[Ninja]

Right... Thanks for all the useful info from you all!

Last question: we agree that 16.8cm is a quarter wave at 446... So, the online antenna calculator says 40cm for 5/8 wave at 446.

I'm thinking that with 1/4 wave being so short at 16cm, i can easily afford to get away with a longer mobile whip, so is there any problems with just cutting a whip to 40cm to hit that 5/8th wave performance?

Or should I stop messing about with cutting down taxi whips and buy one of those Nagoya NA-771 antennas that are 39.6cm long anyway?...

Thanks!

[wa10]

if you want to make a 5/8 you will need some form of matching network to match the high end impedance of the 5/8 to your 50ohm coax,

i made a shunt fed 5/8 for 446 out of a half breed and a pl259 when the cheap 446 handies first came on the market,
i got my calculations for the coil a little wrong but shorting a turn below the tap with a screw gave me a very low vswr.

[8Bit]

Or should I stop messing about with cutting down taxi whips and buy one of those Nagoya NA-771 antennas that are 39.6cm long anyway?

Why limit yourself strictly to HT whips? If you're putting the antenna on the car roof, go large and then use the factory-fitted rubber-ducks when you're outside the vehicles.

Spend a little more and you could buy a decent dual-band mobile antenna with a reasonable amount of gain. Especially, if you look into Super-Gainers or their myriad copies. (Diamond, D-Original, Sharman, etc.)

A Db gain of 5.8 would almost quadruple the 5W output of your handheld to a little less than 20W output from the antenna (precluding any losses in the coax).

Without the equipment to test though, you may be doing yourself (and your radios) a huge favour by putting away the tin-snips. You won't learn anything by spending this extra cash, but you'll end up with something that'll put more of your signal into the air.

I guess for car-to-car "convoy" use, this might seem like overkill but for general mobile use, these are typical of the antennas used in the ham-fraternity. You're unlikely to see a licensed ham using a replacement handheld whip, screwed to a mag-mount any time soon.

446Mhz PMR frequencies are in the UHF band, quite close to the 70cm (430-440Mhz) amateur bands. This band is commonly regarded as "line of sight", where the signal won't travel beyond the horizon without the advantage of height or "Tropospheric Ducting". It also struggles against natural (trees) and man-made (concrete) obstacles and signals in these bands also suffer in adverse weather conditions.

At these frequencies, you need as much height/power/gain as practically possible to make any real distance.

[AshtonMobileScanner]

Isnt 70cm mean the wave length is 70cm. I thought the length of the antenna shudvbe 70cm thats how i use mine. Is this wrong.

Thanks guys.

[8Bit]

70cm is an approximation of the height of a full wave at the closest amateur radio frequencies. All the ham-bands are named by a rounded approximation. 23cm, 70cm, 2m, 6m, 10m, 20m, 40m, 80m and 160m.

As I mentioned in an earlier post, a more precise way of calculating the full height of the wave for a particular frequency is to take the speed of light in free space (usually rounded to 300000000 or 3 times 10 to the power 8) and then divide this by the frequency in Hertz. The result of the calculation is the full-wave height in meters for this particular frequency.

So, for Channel 1 of the 446Mhz PMR "band" 446.00625Mhz, this would be:

300000000 divided by 446006250 (hz) which gives a full-wave height of 0.6726m

As you'll see, the 70cm would be a rounded approximation of 0.67m or 67cm.

70cm is closer to the lower end of the 430-440Mhz ham band as this calculates to 0.6976m or 69cm.

CB Radio is often referred to as 11m, so it could be appropriate to call PMR446 67cm.

Amateur antennas are typically quite wide-band at UHF frequencies, so a 70cm antenna will quite often give a workable VSWR throughout the UK PMR446 band.

[BK]

Isnt 70cm mean the wave length is 70cm. I thought the length of the antenna shudvbe 70cm thats how i use mine. Is this wrong.

A vertical half wave dipole (or a quarter wave monopole over a ground plane) has a radiation pattern a bit like a figure 8 on its side, i.e. it radiates out towards the horizon but not straight up into the sky or straight down into the ground. It also has an impedance of approximately 50j0, i.e the same as your radio. This is what you want.

A full wave antenna has a cloverleaf type radiation pattern, i.e. it radiates up 45 degrees and down 45 degrees but not out towards the horizon. It also has a high impedance so most of the power your radio transmits will just be reflected straight back into the transmitter rather than being radiated. This is not what you want.

[bigbloke]

the antenna length may be ok for a 1/4 wave , but is the cable ?

if you want build quality and gain at UHF then I'd look no further than a panorama ACUHB modular 5/8 collinear on a magmount

http://www.panorama-antennas.com/index. ... uct_id=105
The one that covers 430 is flat VSWR to 449 out of the plastic packaging

yes its a bit pricey - but it will outlast the lot of them. I have 2 here in use since the early 80s - still going

regards

BB

[Bogget]

Hmmm.....to be honest you might as well go on ebay and get either a Nagoya NL-770R for around £15 (this is cheap and SWL spot on for 446) or a Diamond SG7900 much better but cost around £39 (in fact its actually superbe).

I use both of the above, and both are very good of course the Diamond is the best.

The angle of radiation is far to high on a quarter wave for mobile unless your talking to aircraft but the ones above have much lower angle so will give greater range and of course higher ERP as they work as a colinear at 446.

Bogget

[curny]

Gotta agree with Bigbloke. The Panorama antennas are a quality bit of kit. It's what the professionals use.
Well worth the money.

I get all mine from Co-Star antennas:

http://www.co-star.co.uk/prod/781/uhf-5 ... orama.html

Good value and excellent service

Curny