Where does it lead? A bridge to nowhere!

Well,

Following my initial attempts at making a Return Loss Bridge (RLB) last time:

http://g0mgx.blogspot.co.uk/2013/09/return-loss-bridge-where-does-it-lead.html

I've taken some advice; the most common suggestion was that my construction was too slap dash! Could do better and must try harder - reminds me of my school reports!

Here's a remade version of the RLB in quite high-res:

Now this is much neater, however, the basic problem is going to be (so I'm being told) that the wires and the box create a feedline - my construction makes that feedline a load of pants....

Now, I then tried to make an alternative RLB using more normal type of construction techniques:

And that has a slightly worse directivity reading to the one in the box!

So, then I followed another suggestion and constructed an RLB using SMD resistors and this one looks like this:

The observant amongst you will see that the resistors are in fact 52R3 (thats 52 point 3 ohms); these were left over from the power meter project back here:

http://g0mgx.blogspot.co.uk/2012/11/the-power-is-with-me.html

This newly made RLB when boxed measures at 26.3 dB directivity - so this is an improvement and much closer to my target value of 30 dB.

I then decided to rummage further and I found some 100R SMD resistors, and made a third (forth?) RLB using 2 100R resistors in parallel for each 50R theoretical component:

Now this version of the RLB measures as 23dB directivity - so just the same as the first one! Bah - I'll stick with the version containing the 52R3 components for now....

New ham cat's not been helping much:

Fun, egh?

Return Loss Bridge - Where does it lead?

Well,

More fiddling today and I have been making a return loss bridge to help with my "RF Workbench" measurements.

A RLB (Return Loss Bridge) is a device that will help me understand how close to a desired target impedance of 50R things are. And by things I mean kind of anything really, an amplifier, a dummy load, an antenna, a rubber plant - anything. That last item isn't necessarily true - I am just checking you are paying attention.

What a RLB does is allow you to measure the difference between the input power and the power reflected as a result of an impedance miss-match. So the higher the return loss the better the impedance match (unlike SWR where lower numbers indicate a better match).

Here's a simple schematic of an RLB that I have stolen:

So, I've made mine using 2 x 100R resistors in parallel for each of R1, R2 and R3 and the transformer T is wound with 10 turns bifilar on a FT37-43:

Now, to calculate the bridge directivity, firstly you would connect a 50R output signal generator at the test frequency to the "RF" port and a 50R terminated 'scope at the "Detector" port. Once this is set up we need to measure the peak-to-peak voltage reading on the 'scope. In my case I set this up for 7MHz and read a value of 57 mV with the "?" port open circuit. If I now connect a 50R load to the "?" port and take the reading again, I get a value of 3.7 mV. This tells me that the directivity of the bridge is:

20 LOG (57 / 3.7) = 23 dB (ish)

Now, that's a bit pants! I was hoping for at least 30dB. THe first thing I tried was changing the toroid in the transformer "T" - I remade it using a FT37-50 with 10 bifilar turns:

This change hasn't made any difference at all!

The theory of operation is that you would set your signal generator to the frequency required and then first measure the detector reading with the "?" port open and then repeat the measurement with the device under test connected to the "?" port like this:

But given that my RLB only has a directivity of 23 dB ish I think I need to make some improvements. This should turn out to be a useful addition to the shack however, and in my quest to learn more and more about RF measurement, impedance and the like - this has been a good experiment!

Odd, Egh?