CATV Reflection (SWR) bridge: how to construct

[g579]

ha8et

My question is : how to construct a CATV 5MHz - 1000 MHz (perhaps 2000 MHz)
Reflection (SWR) bridge in practice (RFin, RFout, no diodes, only passives).
Reference terminations should be changeable.
Thanks for all help in advance.
g579

 

[flatulent]

ha8et 034

Here is a directional coupler design program. It uses two toroid cores to make the through and the forward and the reflected ports.

Once you get the design parameters it should be analyzed on a SPICE type program with the parasitic capacitors added.

I suspect that you will need two of these to cover the entire frequency range. Alternately, you can calibrate one with different load resistors at each frequency you want to measure.

 

[g579]

swr bridge

Flatulent,

Thanks for the enclosed program. It is a useful one.
But I am looking some bridge configurations where only one balun is needed.
I guess that the bandwith of one balun (even if it consists of perhaps two) should be wider than two separate transformers.
One question about transformer modeling with Spice. How do you model (or where can I get models) for transmission line transformers wounded on "low freq." ferrites ?
Thanks
g579

 

[pewang]

bridge catv

My company has buyed one such reflection meter,its operation basics is based on balance resister bridge.Operation frequency up to 1GHz and reference impedance can vary 75+/-10 ohm.
I will find some detail and post later.

 

[g579]

swr bridge transformer

Could anyone give me some more info on reflection / directional bridges construction details ?
Thanks
g579

 

[flatulent]

swr bridge pollak

For the question about modeling several posts above: You will have to do two separate simulations using just a transmission line in one and the inductor mode in another.

The two transformer method above does not use transmission line type windings. They are two genuine transformers. The impedance levels are low enough that they are quite broad band.

 

[AL0117]

vswr bridge design

Hi,

you will find below a link to a VSWR bridge by HA8ET. The model HA8ET-033B is dedicated to antenna measurement (needs higher power level, since it has a diode detector), but the author has an other model HA8ET-034 which is usefull for measurements with a spectrum analyser with tracking generator. However, this one is not described on the webpage, but as I know HA8ET, if you contact him, he will help with all the documantation. The HA8ET-034 can be easily modified for 75 ohm, so you can use it in your 75 ohm system

hxxp://www.pollak.sulinet.hu/elektro/hidak/hidak.htm

Also some technical details about bridges:

hxxp://www.gie-rf.de/sc1599/Webdownload/GIE6620A_1GHz_Low_Cost_VSWR_Messbr%FCcke_75Ohm.pdf
hxxp://www.gie-rf.de/sc1599/Webdownload/AN6620A_1.PDF

rgds, Al

 

[g579]

bridge reflection rf

Hi Flatulent and AL0117,

Thanks for your input.
Flatulent,
It is clear what you says, but about modeling, the things are not quite clear.
In the low freq.range modeling with inductance is OK. In the high freq. range modeling (with practically no ferrite effects) is also OK. But how to model in the mid range, where the ferrite material features are (at least) ambigous ?

AL,
Thanks for the links, the 2nd and 3rd one is especially useful. It explains a lot of things. Thanks.
g579