EMRFD Message Archive 11159
Message Date From Subject 11159 2015-06-11 11:53:15 ve6jf Return loss bridge
My first project in a long time (ie: late 70's) is a RLB (Fig 7-41). My first objective is to tune antennas and while I think I have it right, I'm looking for feedback to verify that I'm on the right path.
Since I don't (yet) have a spectrum analyser, my idea was to use a signal generator [HP8640B] and a 100mHz scope for measurement at the detector port. My initial measurements at the detector port are 513mV open and 505mV short. So first question, am I correct in my calculations that .505/.513=.9844 and this is 0.14dB for the O/S measurement?
Next with a good 50 ohm termination at the unknown port I see 2.172mV at the detector port which to me is 47dB and a VSWR of 1.009
If I replace the 50 ohm termination with 100 ohm I see 171mV at the detector port. Based on the open value again, .171/.513= .333 works out to be a RL of 9.55dB and a VSWR of 1.999
Tests with other value resistors seem to give rational results such as a 38 ohm resistor a RL of 17dB (VSWR of 1.329)
Throughout all the tests the scope was terminated with a good 50 ohm terminator.
My plan is to take various measurements and plot values in Excel to tune the antennas. Overall is my math ok and my assumptions more or less correct? Also open to any suggestions as to how I may get more accurate results.
11160 2015-06-11 12:10:09 dave081360 Re: Return loss bridge Hi James,There's an easier way. Check this out:Midnight SNA
Scroll down a bit and see the return loss bridge plots.73.Dave Collins - AD7JT
11161 2015-06-11 13:31:10 Loren Moline WA7S... Re: Return loss bridge 11162 2015-06-11 13:32:38 Barry Cromwell Re: Return loss bridge I can verify the SNA is a valuable tool. I Completed the xtal test fixture and sorted crystals for the measurement rx, finished the RLB yesterday and have been checking it out today. Parts from Digikey should arrive sometime this week to finish the measurement receiver.
11163 2015-06-11 14:27:11 ve6jf Re: Return loss bridge Hi Dave,In all my searches I didn't see that. However I initially came *very* close to buying a RigExpert. Two things happened: I realized that a spectrum analyser with a TG and directional coupler would do the job nicely. And most importantly it could be used for many other things. So that cash can now be applied to one in the near future.But also there is some innate drive in me to build rather than buy! :)Thanks &73Jim
11164 2015-06-11 14:34:10 ve6jf Re: Return loss bridge Hi Loren,This is only for 20 and 15m currently. I don't think I will ever go higher than 6m.Jim 11165 2015-06-11 16:03:41 kerrypwr Re: Return loss bridge Your figures look fine to me.
I would guess that you are working at a fairly low frequency as open/short differences tend to increase with frequency, even with the most careful construction.
You have plenty of accuracy; there's little point in striving for more.
My preference is to use a short on the test port to establish a base figure; I think that a good short is fairly easy to make while a good open is quite difficult because of fringing capacitance.
It sounds as if you are about where I was when I revived a youthful interest in RF some years ago.
Consider building an AD8307 power meter; this was an early project for me and I learned a great deal from it; although I now have more advanced instruments I still reach for it for a quick measurement.
Mine is based on Bob Kopski's design;
Separate "heads" for power and RL measurement proved very useful; the RL head has an AD8307 across the detector terminals.
Since the meter reads in dBm, it saves a lot of calculation; just note the open or short reading and the measurement reading and it's mental arithmetic. :)
I did a lot of measurements just as you are doing; it's laborious but I learned a lot because it's "hands-on".
For instance, measuring the parameters of fifty or more crystals with a signal generator and a power meter is fun at first and teaches you a lot but there comes a time when you want to move-up; the knowledge & experience gained from doing it the "hard way" stays with you though.
11166 2015-06-11 16:27:44 DuWayne Schmidlko... Re: Return loss bridge Another option if you can use an instrument connected to a PC are the Chinese SNAs that are showing up on auction and other sites.
There are several of them with different frequency ranges, the one that I purchased was listed as. "NWT70 frequency sweep meter 0.05-85 MHZ with 0 to 50 db attenuation".
The software that comes with it has functions for use as a SNA, Signal generator, Power meter, SWR scanner. SWR also has an option of computing SWR at the antenna after you input info on feed line length and loss .
I put some information about it and and a simple SNA I built on my blog kv4qb.blogspot.com. I have a post on comparing the RLB output with my antenna analyzer, and a adapter I built to use it as a dip meter for finding the inductance of small air wound and toroid coils.
11167 2015-06-11 16:46:31 ve6jf Re: Return loss bridge Hi Kerry,The power meter is an upcoming project for sure.Yes, I built my 1st QRP 40M xmtr back in the mid 70's. I have been QRT for far too many years!I just finished testing both antennas and it all seemed to work out as well as can be expected. Initially my 20M dipole resonated at 13.22mHz with a RL of 17dB. The 15m one was 19.67Mhz with RL of 16.2dB. I tuned each of them and in the end they both show about about the same values but now are where they should be.I ended up with a VSWR of about 1.3 on both antennas. They are not very high, so I don't expect perfection.One thing I did notice doing it this way is that there is a *lot* of noise on the scope trace which made accurate measurement quite difficult. I suppose this is where a dedicated instrument would be a better tool such as the SNA Dave suggested. Or a spectrum analyser with a TG. But when the only tool you have is a hammer, everything looks like a nail. I'm working to change that..73JimVE6JF