That’s quite impressive it could pick it up! That’s a tiny amount of reflection!
I’ll have to see if I can track down that article on probes, sounds like an interesting read 
It always amazes me what could be accomplished with slide rules and brain power.
Edit: Here are the two articles I found, one from '89 and one from 2009. I’ll have to get a subscription so I can have a read
https://www.siliconchip.com.au/Issue/2009/October/The+Secret+World+Of+Oscilloscope+Probes
https://www.siliconchip.com.au/Issue/1989/June/Understanding+Oscilloscope+Probes
1 Like
I think the spec from memory was about 36db. With the long cable anything much worse was re-transmitted as a ghost about 10mm after the main image. The measuring set up was a bit more sophisticated than the simple set up in the video although the CRO image was similar with the echo being right down in the noise. There was some R & S pretty fancy test equipment involved. I won’t go into the measurement technique here as it is a bit lengthy but I you are interested send me an Email I can reply to and I will do my best to describe.
When I was getting the printed version I used to tear out interesting articles so there is a possibility I have it somewhere. Will have a look later.
Cheers Bob
2 Likes
Hi Oliver33
Found the one from 2009. Knew I wouldn’t have thrown it away (bower bird me)
Scanned it for you as PDF, 8 pages at 600DPI to make sure of resolution so file size quite large.
This is getting away from the initial thread subject but is the only way I can get it back to you.
Feel free to add it to your U-Tube video if you want. It is a pretty good instructional read.
File is too big
This is a dropbox link, see how that goes.
I have removed the dropbox link as I am concerned that putting this up on a public forum may infringe some copy write laws or something and I could stir up a heap of trouble.
Cheers Bob
2 Likes
That’s awesome, thanks Bob! I managed to find a way to download it anyway. I’ll send you a direct message about the Rhode & Schwarz(?) test setup you used
1 Like
Hi Oliver
Yes R & S is short for Rhode and Schwarz. They make some test gear that the average hobbyist can only dream about.
The particular instrument we used was known as an Impulse Reflectometer and designated ZUP-1 (I think, a long time ago). I can’t remember if there was an external reflectometer used or not. There must have been some sort of device to rectify the RF for display purposes or it may have been looked after in the instrument. I forget now.
The instrument was a big noise generator with a reasonable amount of power. This was applied through selected filters which passed the energy corresponding to the TV channels. That meant you had a great chunk of broadband energy covering a TV channel shoved up the antenna cable. The incident and reflected signal were displayed on the CRO screen. The CRO gain was increased to display the reflected signal at a usable amplitude and the reference level established. The incident signal is passed through a switched accurate attenuator which has a resolution I think of 0.1db. This attenuator was accessible via a row of knobs on the front. The rest was fairly simple really. The attenuator was increased until the incident display exactly equalled the reference level set earlier on the reflected signal. The attenuator figure was then read as return loss.
This worked fine for an antenna with a single channel. There are several situations where more than one channel uses the same antenna so tuning and measuring these is a bit more tricky. The instrument used here is a Polyscop/Selectormat combination (yes R & S again) with a very nice reflectometer and 50 ohm reference load. This reference is not a connector with a 50 ohm resistor in it but a 1% load with as close to zero reactance as you can get and the connectors are 1% Spinner types. A 50 ohm resistor really is only 50 ohm at DC. The Polyscop part is a sweep generator with a very wide sweep range. The one we had was pretty old with a CRT display of about 10 inches. The Selectomat is a tracking receiver which tracks the Polyscop sweep and amplifies the very small reflected signal to a usable and displayable level. I think this instrument is still available in a more modern form but basically operates the same. Probably at a mind boggling price. Very clever bit of kit really. Displayed VSWR on the forward sweep and frequency response on the reverse. I think that is right. Two different parameters anyway.
In a way it is sad not to be still involved in just using some of the beautiful and very smart items of test gear at our disposal. One HP instrument I remember using to demonstrate to a fitter that 50mm of component lead had inductance and what’s more we were able to measure it.
Hope you find all of this of interest.
Cheers Bob
1 Like
