[Elecraft] Help - Antenna Noise Bridge wanted

Tue Nov 19 23:42:00 2002

Tony,

There is little information about the MFJ204B on the MFJ website. I found a
bit more information on the WB0W website. It doesn't appear to be a noise
bridge, but rather a tunable oscillator with a resistive bridge. It is a
self-contained unit that doesn't need a receiver to serve as a detector. A
noise bridge uses a wideband noise source as signal source that covers the
frequency range of the instrument, and requires a receiver for use as a null
detector. I have found that a carefully constructed noise bridge usually
provides finer resolution and more precise measurements than one obtains
with a self-contained antenna analyzer. I use my MFJ259B antenna analyzer to
give me a rough estimate of resonance and impedance - then rely on my noise
bridge for more exacting results.

Noise bridges were the rave in the late 70's and early 80's but have fallen
in popularity with the arrival of low-cost antenna analyzers. My ham friends
and myself (all engineers) attempting to perfect noise bridges spent many
hours attempting to achieve comparable performance to professional bridges.
Our bridges worked quite well, but our efforts were eclipsed by the work of
Forrest Gehrke, K2BT, when his bridge was published in the March 1983, issue
of Ham Radio magazine. He developed a bridge that used a twisted-pair
transmission line transformer that allowed the bridge to hold its accuracy
over frequency (the transformer was coreless!). He also provided a method
for calibrating out the parasitic inductance and/or capacitance that
contribute to such shifts. The bridge uses very common components, with the
possible exception of a good quality 365pF (max) variable capacitor (there
is crystal radio society here in the states that sells this component).

I built K2BT's bridge and it is the very one I use today. As recent as last
Sunday, I was out back checking the feedpoint impedance of my homebrew 160
through 20-meter vertical. There is slick little way that you can hook up a
DVM to the bridge and directly measure the resistive portion (Ra) of the
antenna impedance (Za). My noise bridge provides the Ra portion to a
fraction of an ohm. Although someone may not need that kind of resolution
for casual antenna measurements, it may be desired when working with low
impedance antennas such as phased arrays and close-spaced beams.

What you may actually be looking for is an antenna analyzer. If so, the
MFJ259B is a popular choice here in the states.

Regards, Thomas - AC7A (Tucson)

----- Original Message -----
From: <[email protected]>
To: <[email protected]>
Sent: Tuesday, November 19, 2002 4:30 PM
Subject: [Elecraft] Help - Antenna Noise Bridge wanted

>
> I'm having trouble getting hold of a MFJ204B in the UK, but they seem to
>be  in short supply globally- Waters & Stanton keep promising delivery and
>then  having trouble getting them from the factory n the US.

> Thanks
>
> Tony
> G7IGG