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) 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?

Joop, PA1JAV

----- Original Message ----- From: "Joop & Tineke Verdoes" [email protected] To: [email protected] 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

----- Original Message ----- From: "Joop & Tineke Verdoes" [email protected] To: "'i8cvs'" [email protected]; "'AMSAT-BB'" [email protected] Sent: Saturday, November 18, 2006 10:35 AM Subject: RE: [amsat-bb] Req. help with making a Coax Hybrid

Domenico Thanks for your thoughts and thanks for the info and drawings you provided some months ago, these are what I am using as a reference.

H'm I have thought about the connectors, but

• I solder the 100ohm terminator directly on one side of the coax. No

hidden lenght here.

• I solder the coax directly on the back of a male-chassis-N connector.

So there is hardy any extra length, at least not the amount that would lower the match by 20-24 MHz (at least I think so!)

• The male-chassis-N connector goes on the MFJ-269, but.....the 75Ohm

part ends at the backside of the mal-chassis-N connector, so from there on it should all be 50Ohm. That's why I like to think the length inside the MFJ-269 doesn't matter. I guess I'll start with pieces that are about 24MHz too long, and see where it ends up when I connect the 4 pieces and then probably have more food for thought :-)

Thanks again Domenico

Joop, PA1JAV

Hi Joop, PA1JAV

OK At one side of the 75 ohm 1/4 wavelenght coax you have soldered the 100 ohm termination and at the other end of the line the coax is directly soldered to the back of a male-chassis-N connector with the front side of it screwed on to the female N connector of your MFJ-269

In this conditions it is hard to say how much the length of a male-chassis- N connector is part of the 1/4 electrical wavelength line and how much of this connector is part of the instrument.

The velocity factor of your TV 75 ohm coax cable is in the range of about 0.75 and for 145 MHz the electrical lenght of a 1/4 lambda is 388 mm while for 166 MHz it is 339 mm so that the difference in the electrical lenghts is 49 mm wich is greater than the lenght of your male-chassis-N connector.

Anyway the coax of your 1/4 lambda line is soldered to the central pin of the female-N connector and the braid of the coax is twisted before to be soldered to the flange and this introduces inductance so that when you unsolder the coax from the N connector the resonant frequency of your 1/4 lambda line rises a bit.

To prevent uncertainty in the measurement of velocity factor when building hybrid rings with coax cable I normally use PTFE 75 ohm coax RG-302 for 145 MHz and semirigid UT141-75 for 435 MHz because the velocity factor for teflon is constant at 0.694 so that without using connectors you can cut the pieces with the PTFE right flat to the braid or right flat to the outside copper tube of the semirigid cable leaving out only 1 mm of central conductor protruding off the teflon at both ends of the line without problems with teflon material soldering the coax at high temperature.

When cutting the 1/4 and 3/4 wavelenght pieces the golden rule is "When in doubt,cut it a little bit short".Do not allow extra bits for what you may think may be part of the line lenght -you are almost invariably wrong but using RG-302 or UT141-75 the velocity factor is invariably 0.694 and without the need to make extra measurements to find the velocity factor you can cut the electrical pieces with great accuracy.

In addition you don't mention the type of 100 ohm terminations used at one side of the coax but at 145 and 435 MHz Allen Bradley carbon resistors should be used for your test because they are less inductive than spiralized resistors.

A better solution is to use a SMD 100 ohm chip resistor directly soldered between the central conductor and the braid or two 100 ohm in parallel if a 50 ohm termination is required.

If only conventional spiralized carbon resistors are available than wrap up the body of resistor with a thin foil of copper recovered from the braid of a coax cable and then solder the foil only at one terminal of resistor so that the capacity between the foil and resistor will partially cancel out the inductive reactance of the carbon spiral.

I hope this helps

73" de

i8CVS Domenico