Return Loss Test Set

The 141T (display) / 8552B (IF) / 8554B (RF) sounds like what you want. It will cover (approximately) 10 MHz - 1.2 GHz, and you can use an 8444A tracking generator with it.

The 8502A and 8502B return-loss test sets are probably usable with an

8554B / 8444A combo, but I can't swear to that. They are meant for use with the 8505 network analyzer. I believe the 8502B is 75 ohms and the 8502A is 50 ohms; you may end up wanting both, if you are interested in CATV work.

Here's the HP Journal issue that described the 141T and its initial set of plugins:

(16 pages, 2 MB)

-- jm

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To measure return loss, what you really want is a network analyser, not a spectrum analyser. Unfortunately network analysers seem to be more expensive. As a starting point you could get a diode detector and a signal generator for the frequency of interest, and a directional coupler. You would also need a known short circuit with the right connector type to determine what 100% reflection looks like on your meter, and also a known resistive terminator to determine what an ideally matched device looks like. I think this is about the minimum setup you need to measure return loss. The accuracy will depend greatly on the quality of the directional coupler. You could substitute a spectrum analyser such as the 141T in place of the diode detector, but it will not bring you a lot of improvement, except perhaps that the level of harmonic content in your signal generator will be less important. If you want to do much better then you really want a Vector Network Analyser (VNA).

If you are only interested in low frequencies then you could look at the N2PK network analyser. (try google) It would be nice if someone would design something similar to cover up to 2.5GHz (e.g. for WLAN antennas etc.), but the first stumbling block is the need for a good signal source. There are suitable mixer chips e.g. AD8343, which could be substituted with little pain into the N2PK type design, but the synthesisers are the hard part, especially if you want the same instrument to cover the range of frequencies down to low RF or audio. A decent 4-receiver 3GHz VNA would be a very popular project if someone came up with a good design.

Chris

How about using a slotted line long enough for the frequency of interest. Or a slotted waveguide if the frequency is high enough.

Bill K7NOM

Brian, The 141T does indeed have tracking generators as part of the "lineup". The 8443 is a counter/tracking gen up to 110MHz (pairs with the 8553 RF plugin), and the 8444 tracking gen up to 1.3GHz with the 8554B RF plugin. Neither are particularly difficult to find.

As-is, they will easily permit the measurement of swept transmission (thru) loss/gain measurements without any additional equipment. However to do return loss you will need a directional bridge of some sort. I believe that there is an accessory 6dB directional bridge that I can't recall the model number of. Or you could use any frequency-appropriate directional coupler with a reference adjustment for the coupling in dB.

Chris, Bill, Scott, Thanks so much for your comments. You're right in that I really would LOVE to have a VNA... maybe one of those Rhode & Swartz (sp?) jobs that sell for about $40K on ebay ;-) It's great to know about the 8444/8554B plugins. I wasn't aware of them, and will now start looking. I did know about the need for a directional bridge or directional coupler, thanks for confirming my understanding of these.

Brian Aase

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Look into the HP 8505. They are in the $1K range, which is a real bargain if you have the space.

-- jm

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Note: My E-mail address has been altered to avoid spam

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hey again Brian, First, a minor point, the tracking generators I mentioned aren't plugins but full rack-width equipments, probably 5" tall. Someone else mentioned the 8505 VNA...which is the same vintage (up to

1.3GHz like the 141T), but *by itself* it's just a multi-channel tracking receiver. To make up a VNA you still need a sweep generator and a bridge, which were conveniently manufactured as the 8501 (sweeper) and 8503 (s-parameter test set; a fancy name for a dual-directional bridge). $1k or so sounds right though.

If you need more bandwidth there's the 8405 VNA system, which depending on the pieces works from 110MHz to 18GHz. That also is a multiple-box deal, but with a lot more possible combinations involved. You can get set up for .11-2.4GHz VNA measurements with that lineup for right around $1k too, maybe a bit more, but with the flexibility of going further up if microwave is your thing.

Scott, thanks again! I'll research all of these and see what I can come up with at a reasonable (okay, cheapskate) cost. Would anyone care to offer suggestions of good equipment sources... other than the "T" company and the "E" website? ;-)

Brian Aase

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