Batboard

Ok have access to a great site with an existing 8 bay UHF 460-480 split I need it to work on 449.000. Is there a way to tune it?
I cannot change the antenna or modify it. Its either make it work or I cant use the site. So Any Ideas.
Thanks in advance
Doug

Put an isolator on your transmitter and hope for the best. Will you be running tx & rx on that antenna, or will your rx come off the site rx multicoupler (if there is one)?

Running RX and TX no no rx multicoupler
Doug

Rats. Now it's darn near impossible with off the shelf stuff. An isolator would let xmit rf pass to the out of band antenna, and any reflected power would go to a dummy load. But, so would rx. Time to pull out the ARRL books. There's a recent thread on using an out of band antenna. Since this is a folded dipole, it does have a phasing harness. You wouldn't lose too much performance using this T band antenna at 440. But, matching at the bottom of the feed line is not as effective as matching at the base of the antenna. Most tuning ckt designs are for very low freqs. I don't think I've ever seen one for UHF. The solution may be as simple as a stub Tee'd at the base of the antenna, or a small loop of coax. You will have to experiment.

First silly thing to check: what is the mismatch? It may be low enough to not worry about.

TX > ISOLATOR > DUPLEXER (ANTENNA) < RX

Granted it isn't ideal.

The first thing to check would be the degree of mismatch as Wow suggested. Assuming that it's poor enough to require correction, buy a coaxial pi net impedance matcher from Telewave that will allow you to tune out the reactance so your transmitter will see 50+j0 ohms(no reflected power). The pi net is bidirectional so it'll pass transmit and receive in opposite directions. And it's a lot less $$ than circulator/load or isolator combinations. Then hope for the best with regards to the efficiency of the antenna at 449.0 Mhz and accept whatever you get.

Or realize that systems are usually built to accept some level of mismatch. My county RACES repeater on 146 is making use of a pre-installed DB264 150-160 split... SWR is 1.4:1, so with 125 watts drive I'm only dealing with 3.4 watts reflected. Just works - and the DB264 and 1-5/8" Heliax was provided free for my use, already installed on the tower and ready to go!

tvsjr wrote:Or realize that systems are usually built to accept some level of mismatch. My county RACES repeater on 146 is making use of a pre-installed DB264 150-160 split... SWR is 1.4:1, so with 125 watts drive I'm only dealing with 3.4 watts reflected. Just works - and the DB264 and 1-5/8" Heliax was provided free for my use, already installed on the tower and ready to go!

Bet it's a TKR with an amplifier on it... ZING! (nothing wrong with that btw)

Nope the SWR is 3:1 ont he antenna at 449 1: at 465mhz. I will look into the impedence match that sounds to be the most hopeful cure.
Doug

How about this Telewave product Telewave TZM-450
E-Z match transmitter to load, 400-512 MHZ, Could this be what I'm looking for?
Doug

I couldn't find a datasheet for the TZM-450, but the description in Hutton and Tessco suggest that's the doodad you want for matching the transmitter to the antenna at the indoors end of the feed line. How well it will pass rx needs to be determined. For $180, it is worth a try.

Ya, that's the matcher type that you want. EMR Corp makes one too. Insertion loss on either one is quite low at less than .2 dB. The EMR 6550/Z is rated for a little less power and 300-650 Mhz but it's $145 list:
http://emrcorp.com/catalog/PDF/AA_LM_LP ... 0_1000.pdf

So, how would he tune that - for maximum forward, or minimum reflected? I'd love to see the instruction sheet. Could be a handy thing to have when caught in a pinch.

The 1:1 at 465 suggests the antenna and line are okay. The 3:1 at 449 makes me wonder what it is at the antenna. If there is just 1db of loss in the line, it would be 3.5:1 (~30%) at the antenna knocking 1.6db off his gain figure. That might come in handy to keep his erps down, or it might not. Again, certainly worth a try.

You could tune as you suggested for minimum reflected power. I'd use an antenna analyzer to get a swept return loss plot vs. frequency or a service monitor with tracking generator and return loss bridge to get the return loss somewhere in the ballpark without risking the transmitter PA health. The EMR pi net can be supplied by Tessco, covers a wider frequency range than the Telewave TZM-450, and 250 watts is probably more power than the OP is dealing with.

Thanks guys I ordered the TZM today of course they don't stock it. But @ $139 from Tessco. Ill give it a shot. I have one of the vocom 350w amps not that I'm going to use it I have a 100w Henry that I want to use but with the TZM I have the option. I have a portable spectrum analyzer and service monitor so hopefully ill get this working. If your near Scranton Pa its going to be on 448.775. 141.3 around Elk at 2600ft.
So we will see cross my fingers.

d119 wrote:Bet it's a TKR with an amplifier on it... ZING! (nothing wrong with that btw)

Find me a Moto repeater of comparable quality for $1K and I'd be all over it

tvsjr wrote:

d119 wrote:Bet it's a TKR with an amplifier on it... ZING! (nothing wrong with that btw)

Find me a Moto repeater of comparable quality for $1K and I'd be all over it

I've recently seen some MSF5000 units for less than $1K, lol.

mirage wrote:Running RX and TX no no rx multicoupler
Doug

Kinda following this. You are running TX and RX into the same antenna? Duplexer? Or separate antennas with some vertical isolation?

Dave