----- Original Message ----- From: "Joop & Tineke Verdoes" joop.verdoes@nameplanet.com To: amsat-bb@amsat.org Sent: Friday, November 17, 2006 1:06 PM Subject: [amsat-bb] Req. help with making a Coax Hybrid
OM's I am at the moment attempting to make a 2m and a 70cm Coax hybrid divider/combiner. I get interesting results. I have made 3 1/4 and 1 3/4 lamda pieces of 75 Ohm coax.
- I included the velocity factor (after measuring this)
- I tested each by using them as a 1/4 or 3/4 impedance tranformer 100
Ohm to 50 Ohm, my MFJ 269 sees each of the pieces terminated with 100 Ohm, nicely as a 50 Ohm thing, right on the frequency I want (146 MHz)
Hi Joop, PA1JAV
When you test the above 75 ohm coax 1/4 and 3/4 electrical wavelenght long transmission lines as impedance transformers using a MFJ269 you includes the lenght of a coax connector in the above line.
In this condition with connector included you found a lenght of coax + connector by wich at 146 MHz the impedance transformation between 50 ohm and 100 ohm occurs because the 75 ohm matching line is actually 1/4 wavelenght long resonant at 146 MHz
As soon you removes the connectors and solder the 4 piece in to a ring to form an hybrid combiner/divider than the 1/4 and 3/4 lines becomes shorter for 146 MHz by the lenght of the above connectors so that the hibrid resonates hygher as you reported at 166 MHz.
I solder the 4 pieces into a ring, terminate with 50 Ohm at 3 ports and connect the MFJ at the "TRX" port. The thing works (ie shows 50 Ohm with and swr of 1.0).........but at 166MHz. At 145Mhz, the swr is 1.2 and the impedance shows some reactance. Before attempting to make wild guesses which leg of the hybrid I am going to cut/extend, would like to ask whether there is someone who can point me in the right direction. The coax cable is 3c-2v tv 75Ohm coax. It appears that the pieces are a bit too short, but this doesn't agree with my test of each leg seperately as an impendance transformer.
Any suggestions?
The pieces are now a bit too short because you have removed the lenght of the coax connector.
When cutting a resonant lenght of coax cable the lenght of any coax connector must be included in it because the connector becames actually part of the resonant line.
Joop, PA1JAV
73" de
i8CVS Domenico