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Author Topic: A PrettGood RLB  (Read 617 times)

Posts: 150

« on: March 29, 2019, 09:16:29 AM »

Back in early November 2018, I decided I wanted to get back to tinkering on the bench.  After 5+ years of neglect, it's amazing just how much stuff gets placed "temporarily" on an available surface.  After a week of digging out and organizing and a couple of upgrades later, I was back in business.

Trying to make a long story short, I had a desire for a Return Loss Bridge for some tinkering.  I naively assumed getting a decent one would be quick and simple.  Aha! Not so.  Finding one to buy for what I wanted to spend turned out to be opposing goals.  Could I build one?  Yes, sort of.  Building one to work well at UHF involves tiny enclosures and smd's.  20 years ago, maybe.  Now, no.

If I was willing to spend around $300, I could get a used Eagle or one from Masters Comm.  A new Eagle goes for $800.   These offer very good directivity specs over a broad frequency range.  Directivity is a key spec for rlbs and couplers.  It's not as critical of a spec for forward power measurement, but it's the key to accurate reflection measurements.  (See the app note at Bird RF, Straight talk about directivity).

So, being unwilling to spend more than $100, what to do?  Roll the dice.  Ebay!  There was a number to choose from - most were open pcbs with sma connectors.  Open boards are not a good idea at VHF and UHF - the sma's I could live with.  Then one vendor offered his in a nice box - with sma's of course.  It's a fellow in the Ukraine who sells as 60dbmcom  (Mr. Kilowatt, maybe?).  I ordered, it shipped and 4 weeks later, it arrived.  Visually, it's top notch.  The case appears to be a milled aluminum piece and the bottom is secured with 6 screws - it should be pretty RF tight.

I've had a chance to run some tests on it and for what I want (and what I was willing to spend), it fills the bill.  These are the numbers I got when testing it using a very good terminator (49.7Ohms). I normalized  the TG for each range with the plane at the end of the test cables.

1 - 30Mhz           RL= -47dB
30 - 60MHz         RL= -44dB
60 - 160MHz       RL= -39dB
200 - 400MHz     RL= -32dB
400 - 500MHz     RL= -29dB
500 - 700MHz     RL= -28dB
700 - 1.4GHz      RL= -26dB

The reflection software in my SA offers a Calibration to an Open prior to the measurement, so the directivity of the rlb is close to the RL numbers.

The bridge is stated to work to 3GHz and it may well do so.  My testing ended at 1.9GHz when the RL dropped to -20dB.  This level indicates a low directivity and most likely ambiguous RL numbers.
So, if you're in the market for an RLB and your frequency range stops at the 23cm band, this one is not a bad choice at $41.00 delivered.

I plan to do more tests on it and with it.  I recently took delivery of the DG8SAQ VNA 3c.  I'm still circling that puppy.  It's going to be a steep climb.  Getting back into tinkering is always a good reminder of what you don't know and how much of what you did know, you've forgotten.

Good tinkering and good day, eh?

And you can't edit the subject after you hit "Post"


Posts: 1059

« Reply #1 on: March 31, 2019, 08:06:11 AM » to save you looking.

Peter DL8OV

Posts: 157

« Reply #2 on: April 01, 2019, 10:04:45 AM »

I purchased on of those masters communications RLB1500 bridge
and it works perfectly and is most likely the best price to be found
on one that works as it should.
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