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+---- Thread: How long do I cut the limbs for PMR 446? (/showthread.php?tid=6398)
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How long do I cut the limbs for PMR 446? - NorthernRaider - 19 December 2013
I've bought this dipole center section to make a home made PMR 446 antenna
http://www.ebay.co.uk/itm/160853656931? ... 1439.l2649
But I do not know whether I include the threaded part of the limbs as part of the 160mm long limbs or does the 160mm length start AFTER the threaded section ?? IE 160mm ant length plus 12mm for the threaded section??
Oh and please may I ask what is the best type of metal to use for the limbs, Copper?, Brass, Alloy?, steel ??
Can you guys advise please.
RE: How long do I cut the limbs for PMR 446? - Lightspeed - 19 December 2013
Difficult at many levels NR :-)
First this dipole centre is really for 3/8" threaded antenna elements, like DV27s for CB. It is a huge lump for use on 446Mhz.
I'd measure from the point at which the dipole leg ( threaded section ) meets the contacts on the SO259 plug.
Wire used for the antenna elements makes virtually no difference.
BUT.... I would strongly suggest getting yourself an SWR meter that is rated for 446 Mhz to check for correct matching. At UHF frequencies, adjustment for matching is extremely fine.
Like on CB, a bad SWR match from the antenna will damage / kill your transmitter.
Maybe it'd be easier to make a dipole following this instruction:
http://www.delboyonline.co.uk/pmr446/446_2003/pmr446_dipole_antenna.htm
RE: How long do I cut the limbs for PMR 446? - NorthernRaider - 19 December 2013
(19 December 2013, 14:54)Lightspeed Wrote: Maybe it'd be easier to make a dipole following this instruction:
http://www.delboyonline.co.uk/pmr446/446_2003/pmr446_dipole_antenna.htm
Ta muchly LS, Dellboys DIY version is what I'm already using mounted up in my loft but even with that and using 4 watts I'm only getting about 3/4 of a mile ??
Hmmmmm curiouser and curiouser, just been moving the home made ant around the loft, I still can only just pick up NR junior calling from the bottom of the village bout 3/4 mile away and further and I get nothing cept the click of him triggering the PTT button, but I can hear truck drivers from the quarries on the A1M 3 miles away in another direction !!!!!!!!!!!!!!!!!!!!!
RE: How long do I cut the limbs for PMR 446? - CharlesHarris - 19 December 2013
At UHF frequencies, the type of coaxial cable used, and the type of connectors used make a big difference. The so-called "UHF" connectors PL259 male, and SO239 female, are not constant impedence above about 150 Mhz, and result in significant losses. Also, the common RG8 foam coax typically used for CB radio, has excessive line losses for UHF. I'm not sure which cables are available yo you in UK, but in the US we run Times Microwave LMR400 cable for anything above 150MHz or the equivalent Belden 9913, and would prefer to run only N-type connectors, if possible. On modern UHF portable transceivers the SMA type connectors also work well.
If your antenna is in your loft, the presence of any metal within about a wavelength of the antenna can cause coupling which affects its resonant frequency and disrupts the radiation pattern of the antenna. If you can find a local radio amateur with both SWR and a field strength meter, it may be helpful to do some trouble shooting.
The US amateur 70cm band includes your 446 MHz portion. It is commonly used here for FM simplex and repeater operation. It shouldn't be difficult to find a US-band 70cm antenna which is resonant on your PMR frequencies. Most commercially made antennas will have a gamma-match stub to allow specific tuning for best SWR on your working frequency, but also have sufficient band width to not exceed a 2:1 VWSR over the 5 MHz repeater splits used on the US FM phone band. An end-mounted 4-element yagi with a boom length of 1 wavelength should provide about 8dB of forward gain if properly set up.
RE: How long do I cut the limbs for PMR 446? - NorthernRaider - 19 December 2013
Charles, thanks very much for taking the time to draft this in depth reply, I got to be honest and admit I undersood about 1% of it, I'm a technophobe as LS will testify
RE: How long do I cut the limbs for PMR 446? - Lightspeed - 20 December 2013
Sounds like NR Jr is trying to reach you from the village itself, and is transmitting with buildings blocking line of sight to you. The trucks you can hear 3 miles away are probably in a clearer line of sight to you?
What Charles is saying is that using wrong connectors and coax cable at 446Mhz will absorb a lot of signal, both on transmit and receive. He's right and I didn't pick up on this. The dipole centre you have purchased has the wrong type of coax connector and is suitable only for frequencies up to approximately 144 Mhz.
How long is the coax between your transmitter and your antenna?
RE: How long do I cut the limbs for PMR 446? - NorthernRaider - 20 December 2013
The 160mm Dellboy DIY Dipole coax is 10 meters long its RG58U 50ohm using a pl259 to SMA male adaptor to connect it to the Baofeng.
I'm on a hilltop BTW and NR junior was at the bottom of the hill, buildings are in the way.
RE: How long do I cut the limbs for PMR 446? - Lightspeed - 20 December 2013
Your 10m of RG58 and the connectors are probably eating around 30% of your signal.
So that's not the main problem you are getting.
Most likely NR Jr needs to wander around a bit to find some sweet spots of transmitted signal between your base station and the village.
Another approach to finding out how your base station signal is getting out is to create an automatic beacon. This is easily done with certain cheap MP3 players. see http://www.youtube.com/watch?v=yBD1jgmJjrc With this operational, you can wander around the local area monitoring the transmitted signals. This will give you a map of viable transmit and receive locations. You will find that on occasion just moving a few feet will make a difference.
RE: How long do I cut the limbs for PMR 446? - NorthernRaider - 20 December 2013
(20 December 2013, 11:05)Lightspeed Wrote: Your 10m of RG58 and the connectors are probably eating around 30% of your signal.
Bugger, I may be able to do some major cable rerouting and get it down to say 5 meters would that cut the loss of signal by half?
Would more elivation on the ant overcome the problems broadcasting south co thats the direction I need to transmit most as NR junior and Mrs NR work or school that direction??.
Starting to think i need a burner and a much higher antenna to reach the places I need to reach.
BTW theres little or no metal in my loft to interfere cept the neighbouring CB ant sat on a 2 ft square sheet of metal earth plane, would moving the PMR ant outside gain much??
Ideally I really would like to keep the antennas in the loft for Opsec and MTCE issues.
RE: How long do I cut the limbs for PMR 446? - Lightspeed - 20 December 2013
Yes, halving the coax length will half the losses from it, but not the insertion losses of the connectors.
Getting the antenna higher and in the open is always a good idea, and for sure will improve things a good deal.
This antenna has good reviews. It appears to be tuned specifically to PMR446 band, so may be worthwhile considering. I have not played with one myself, so only have web reviews to go on.
Re burners: don't forget that your signal is being attenuated both ways. If you install an amplifier, then you'll need to install a receive pre-amplifier as well. Make sure that both are tuned to 446 Mhz
Personally I'd forget about amplifiers and pre-amps and concentrate on getting a well tuned antenna high and in the clear.
I'd feed this with as short a run as possible using the best coax I could afford.
Connectors should ideally be N-Type.