Measuring Antenna Return Loss (SWR) with HP 141T Spectrum Analyzer and 8721A Directional Bridge

This video shows the use on a 1970's era HP Spectrum Analyzer, capable of measurements to 110 MHz, to measure Return Loss of an antenna.

The 141T Spectrum Analyzer system has the 8552B IF and 8553B RF Sections (plug-in's), and is connected through a special cable assembly, HP p/n 08443-60009 (see http://www.glkinst.com/cables/cable_pics/08443-60009.jpg) to an HP 8443A Tracking Generator (early nixie tube version) The 8443A allows the operator to determine the frequency of any position along the trace via a marker and also provides the incident RF signal to the DUT (antenna in this case) through the 8721A Bridge. This was state of the art for that era and how network analysis was done using this 141T Spectrum Analyzer. These particular units were all built at HP in Santa Rosa, California.

The 8721A Bridge is used to seperate the reflected signal from the incident signal and thus measurement of the reflected signal's amplitude changes over a frequency sweep. See HP Application Note 150-3, available at the HP Memory Project Website, at http://www.hpmemory.org/ressources/resrc_an_01.htm for more information.

Note: the 8721A bridge labels appear on both sides but differently. The connection label LOAD is SOURCE on the opposite side, INCIDENT is actually REFLECTED oppositely, and SOURCE is LOAD on the opposite side. I had it setup to measure REFLECTED but that side is facing the table! Darn! Oh well!)

The SA is set to 300 Mhz bandwidth, .5 MHz scan width per division, input attenuation at 20 dB, and a scan or sweep time per division of 5 milliseconds. The persistence capability of the SA CRT, allowed the operator to set very slow scan times for greater resolution and improved accuracy of the amplitude measure and retain the trace marker results as it passed from one side to the other. Note the voltage fluctuations on the oscillosope as each sweep occurs.

The reference line is the top graticule of the CRT at 0 dB. The return loss at a 50.172 MHz ( 6 meter band ) with a 102 foot (31.0896 meters) G5RV antenna was about -58 dB (VSWR = 1.00252102460553) assuming the analyzer is spot-on in calibration. A Palstar AT2K antenna tuner was used to tune to peak resonance.


Return Loss :
Return loss is the logarithmic ratio of reflected-to-incident voltage for a device under test. Return loss can be derived from the linear reflection coefficient (Gamma) using the following equation: RL=-20 log|Gamma|
A return loss of [infinity] dB corresponds to no reflection, while a return loss of 0 dB corresponds to a cmplete reflection.

Voltage Standing Wave Ratio (VSWR) :
The SWR or Standing Wave Ratio is the ratio of maximum to minimum standing wave voltage. An SWR of 1 corresponds to no reflection, while an SWR of (infinity) correponds to a complete reflection.

Enjoy!

73!

Category:

Science & Technology

Tags:

• radio
• HP
• Hewlett-Packard
• 8721A
• 141T
• 8553B
• 8552B
• 8443A
• ham
• shortwave
• VSWR
• SWR
• Agilent
• spectrum
• analyzer
• network
• reflection
• RF
• frequency
• return
• loss

[radiodad2008]

HP and Agilent today, makes some of the finest measuring equipment ever placed on the market. The quality is required because to measure something, it has to be good and extremely accurate. To get that is not easy with RF and Microwave. Thanks for the kind comments. The 8555A is a fine plug-in and I am lucky enough to have one also. One the the lead engineers on its development is a friend of mine. I am so lucky to have friends like him.

[radiodad2008]

Yes, I agree. I had just snapped this quick video after running the test, first time I had actually used the 8721A in the shack. It was not intended to be a step-by-step how-to video, but thought I'd put it up anyway. I'll work on something over the next couple of weeks more informative. Thanks for your comment WA5CVI. FB!

73,

K6SRO

[nlimchua]

@radiodad2008 Do you have the 8406a comb generator? know how to use it?

[nlimchua]

I also have a 141t with the 8553, 8554 and 8555a modules. I like playing with it once in a while. Have the 8406a comb generator too (but still need to learn how to use it!). I remember when I first got this, the 8555a had a bad first stage input mixer module, fortunately found a spare to replace it with, works great. As old as this unit it, I can't help but be impressed by how much quality went into the design and construction - Made in USA!!!

[WA5CVI]

Would have been MUCH MUCH better if there had been something informative about it...if someone could LEARN something from watching it!!!