Topband: Antenna Measurements

[John Kaufmann]

On Thu, 14 March 2002, "George & Marijke Guerin" wrote:
> A General Rado Bridge is the best way to truly find out what the antenna
> impedance is at various frequencies. 

A few comments on the General Radio bridge:

I own one and I agree that it really is the most accurate instrument for antenna impedance measurements.  For those of you who are not familiar with it, the G-R bridge is an old model (probably dating back to at least the 1960's?) that has long been out of production.  You can find them on the surplus market, but last time I checked, they were not cheap.  Over ten years ago, I had been looking for one for a long time and ended up paying $600 for one.  I thought that was an outrageous price, but recently I've seen them going for around $1000.  

The G-R bridge is a completely passive unit, which is virtually immune to signal overload, unlike the most expensive and modern lab gear from HP, etc.  The accuracy specs on it are very good--around one or two percent as I recall.  It is also a pain to use because it requires an external signal generator and receiver. Furthermore, it is completely manual, and requires a lot of dial twisting and fiddling to get even a simple measurement. All this makes for a big, bulky test setup that you definitely won't be taking up the tower!

I've done spot checks on the accuracy of my MFJ259B against the G-R bridge.  The MFJ is generally pretty good at measuring complex impedance (R+jX) when the load impedance results in an SWR of roughly 2:1 or less.  However, if you have a load with X much bigger than R, the MFJ generally will not read the R value very accurately.  You cannot, for example, measure the Q of a coil with any accuracy from a R+jX measurement on the MFJ.  Similarly, if R is much bigger than X, the X value generally won't be read correctly either, unless it's a pure resistance and X=0.  This doesn't affect the SWR measurement accuracy very much, though, and that's what most people care about.

The advantage of the MFJ is its small size and ease of use.  It is reasonably accurate for most amateur work with antennas whose SWR is not too high.  However, if you demand high accuracy, you really need a lab instrument like the G-R.

73, John W1FV