Super Simple Antenna Impedance Bridge

[N1BCG]

I came across this circuit online and instantly admired its simplicity and usefulness. Its origin is credited to an ARRL publication "EMRFD (page 7.24, diagram 7.43)".

Easily made from parts drawer components, it's a sensitive diode detector with a meter that compares the voltage balance between a reference load (50 Ohm, non-reactive resistor) and the test load. The more similar the load is to the resistor the lower the reading on the meter. Any differences in impedance or reactance show as a corresponding voltage.

I use a VOM for the meter and a signal generator as a source as it only requires as much drive to deflect the meter enough to adjust for a null. The only changes I made are the addition of an SPST switch that disconnects the load under test in the "CAL" position for setting the drive level to full scale on the meter and using an 1N34 diode which I had on hand. I also omitted the pot since my signal generator has a variable output. Careful component layout and selection of diode will allow operation into the VHF range.

Paired up with an old skool VFO, this is a fantastic device for tuning up your antenna, generating virtually no QRM, or experimenting with new antenna designs.

[WA4WAX]

Nice!

That is just an Antennascope in disguise.  Please see '56 or '59 (?) West Coast Handbook.

1N34A fine for HF; use hot carrier diode and UHF wiring technique for higher.

BTW, you can excite the beast with a grid dip meter as shown in the Antennascope article. 

Matt :-)

[W1ITT]

Make certain that the VFO or signal generator has a clean output.  Harmonics can give results that will be confusing.  I have network analyzers and a vector voltmeter that all have their place, but little quick-and-dirty units like this can be valuable.  The test equipment that's handy is your pocket is better than the wonder box sitting on the bench miles away.
I'm also a fan of  noise bridges.  Keep your eyes open at hamfests for the Palomar noise bridges with both R and X dials.

[N1BCG]

Make certain that the VFO or signal generator has a clean output.  Harmonics can give results that will be confusing.

Ah! A great point I meant to include... If you just can't seem to get a complete null, try swapping in a 50 Ohm resistor as the test load. If the null becomes deep (meter goes to 0) then your signal source has harmonics. I discovered this by using this method. An antenna will reflect harmonics (for the most part) while a resistor will not.

I used BNC connectors on my bridge for I/O and the reference load, which is simply an old networking resistor in a BNC plug.  Very convenient!

[K1JJ]

Make certain that the VFO or signal generator has a clean output.  Harmonics can give results that will be confusing. 

Yep. That's why the MFJ-259B antenna analyzer eats batteries in an hour or so of continuous use.  They run  the internal VFO in class A to clean it up so that harmonics are way down.

A dirty VFO gives readings similar to running a 75M coax fed dipole on 40M and then measuring the swr.  Makes sense.

T

[WA4WAX]

Here is another approach.

Scope required.  Use non-inductive 50 ohm Caddocks, short PC traces, etc.  Use a good 50 ohm load (Pomona sells some nice ones.)  Not for field use unless you have a portable scope.

Kudos to W2AEW.

https://www.youtube.com/watch?v=eYN7dhdt1Dw