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April 17th 06, 01:39 AM posted to rec.radio.amateur.homebrew

wrote:

I have just built an HF return loss bridge according to the info in
"Solid State design for the Radio Amateur". While it works ok, I was
hoping for a bit better performance. Clearly the "balun" doesn't have
enough inductance to operate well below 10 MHz or so (10 turns bifilar
#30 AWG enamelled wire on Amidon T23-43, as specified in the book -
twisted pair, which isn't specified one way or the other in the book).
This can probably be largely fixed by using a larger ferrite core
(T37-43's available in junkbox). I am interested primarily in the
1.8-50 MHz range, though I wouldn't complain if it worked on 2m too.

But even at 30 MHz a reasonably good microwave 50 ohm load gives only
about 28 dB apparent return loss...Not bad, but I might have expected a
bit more.

And an open and short give about 2 dB different signal levels at 30
MHz. That is with big pads (20 dB attenuation) on both the signal
generator and detector. The detector is an HF receiver with a step
attenuator used to maintain a constant S-meter reading.

The circuit is built on a small PC board using construction techniques
typical for the UHF or low microwave range (except that the test port
connector is an SO-239), and is enclosed in a shielded box. Each of
the three 50 ohm resistors is made of two 100 ohm 1206 chip resistors
in parallel and measures between 50.0 and 50.3 ohms at DC.

Has anyone with experience with this circuit any suggestions for how to
tweak it for best performance ? What accuracy level have you achieved
? Do you know where I might find an error analysis for this circuit ?
Or if I am to think about errors myself, does anyone know how to model
the balun in SPICE ?

73,
Steve VE3SMA

Steve,

I assume you don't intend to run to much power thru' your bridge.

One of the best and "homebrew friendly" ways is to wind a ferrite based
directional coupler. I assume you intend to work with the usual 50 Ohms...

I used a small rf ferrite trafo from Coilcraft. It's a SMD sized two hole
ferrite and by ripping out the original transformer wire and replacing it
with the wiring of a DC, I got a nice DC for up to 1000 MHz.

The range of toroidal couplers can span two decades of frequency and 2MHz to
1200 MHz is not impossible.

I normally wind the DC with a 1:3 to 1:5 N1/N2 ratio. That gives me about
0.5 dB insertion loss and about 10 dB of coupled port loss.

As long as the copper wire only touches the ferrite on the corners ( as
little as possible ) the linearity is surprisingly good and I can expect a
return loss of 25 to 35 dB - wich is enough for most aerial measurements.
I'm using these DC's for measurements between 1 and 500 MHz with good
accuracy.

I suggest you look for :

RF Directional Couplers by Michael G. Ellis, Ph.D.

Cheers

Dan / M0DFI

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