Tektronix 491 Spectrum Analyser

[Psi]

Hey, does anyone have experience with the tektronix 491 10mhz-40ghz spectrum analyser ?

Does it fall into the category of old but very good or just old ?

I'm looking for something that can do the 5.8ghz band for testing/comparing antennas/transmitters and may have the opportunity to get one.
Not sure how much they want yet.

What do you think a working 491 is worth?

[Hydrawerk]

It is 1960's technology... http://www.radiomuseum.org/r/tektronix_spectrum_analyzer_491.html

[G0HZU]

How much is the 491? If it is ultra cheap and you just want to do very crude signal testing across a test site then it might be good enough...

Otherwise my advice is going to be quite harsh in that I suggest you forget that this analyser even exists. I don't mean to be disrespectful to the team that designed it maybe 50 years ago but the user interface looks seriously weird to me and it suggests that the performance and versatility will be extremely limited.

If I wanted to buy a bargain basement 6GHz spectrum analyser for doing extremely simple testing of aerials/signals across a test site then I'd probably look at a 22GHz HP8559A or a big old HP141 based analyser from HP with a suitable plug in for 6GHz. The 141 mainframes look a bit old and hideous with a fairly murky CRT but the 141 is going to be streets ahead of that 491 in terms of usability. But I guess you have to find one for sale.

All of these ancient analysers are going to be a bit flakey when used and they could have worn out or intermittent controls and switches.

But maybe you can live with this for the type of testing you are doing?

[Psi]

Fair points, i will keep looking

[G0HZU]

I did also download the specs for the 491 and it only has a 40dB log range on the display. There were loads of other limitations with it and odd aspects of its design compared to a conventional analyser.

Perhaps its only redeeming feature in 2014 (in terms of purchase) is that you might be able to sell it on to a collector if it is in good condition and you can get it for a very, very low price. Otherwise it really is one to forget

[edavid]

Just old

An HP141 has much better performance.

BTW, good luck making any kind of repeatable measurements at 5.8GHz.  That is definitely a black magic frequency.   You might be better off treating the RF as a black box, and making measurements on the digital side.

[Retep]

A teardown of that beast would be interesting, I wonder what it took to get into the 40GHz territory in those days.

[edavid]

A teardown of that beast would be interesting, I wonder what it took to get into the 40GHz territory in those days.

It's pretty low-tech, but you can read all about it, see section 3:

http://www.liberatedmanuals.com/TM-9-6625-2467-15.pdf

[JoeyP]

I do have specific experience with that beast, and can tell you that the 491 was one of the worst products Tek ever produced. A truly awful analyzer (even when you could keep it running).

They did a dramatic redesign starting with the 492. The 492 shares essentially nothing in common with the 491, and it really deserved much more than a single digit increment. Pretty much everything they produced for decades after were just incremental improvements on the excellent 492 platform.

[sv3ora]

I know that this is an old topic but I wanted to share my experiences on this of one reads it in the future.

The Tek 491 has limitations indeed. But it is unfair to say that this was the worst product of Tek ever designed and more unfair to compare it with the 492 which was released in the 80s. The 491 is a 60s technology.

I think that despite the limitations, going to 40GHz at that time was rocket science. The BUILD quality of the 491 is amazing (real collectors value), everything is rigid and metal (including the front panel). The machine inside is transistorized mostly (apart from the oscillators and the CRT) and there was a Tek kit to replace the main planar tube OSC with a solid state version. We are talking about transistors and gold plated PCBs in the 60s so this could possibly be quite expensive. I do not find many build cost compromises on this machine really. The P7 remanence "memory" CRT is also amazing to look at is you are a collector. I think the prices that it is usually shown on ebay are more than fair, if taking into consideration the high prices of many junk equipment sold there.

[Uky]

AFAIK:

The 491 is a 2nd LO swept type analyzer. No wide band sweeps.
It has no RF attenuator (!) RF input levels should be i sub -dBm levels.
It has no calibrated vertical scale factors whatsoever.

In a competitor analysis document released for internal use by HP, when comparing the 491 with their 141-series
analyzers, HP described the 491 as a "Spectrum Monitor".

It is a collectors item as it is a nice example of craftsmanship from when it was manufactured
and it does well with the cover off. But its performance are inferior to that of HP from
the same era.

Having restored more modern HP and Tek Analyzers, I must, as a Tek afficionado admit that Tek
was never able to fully compete with HP. Sure the 492 and some later models including 275x
offers useable performance but eventually, Tek gave up and left the Spectrum Analyzer market
completely around 1990.

[pascal_sweden]

What would be a fair price for the Tektronix 491 spectrum analyzer?