[Antennas] RE: VHF/UHF Noise Bridge Question

[Barry L. Ornitz]

Gary Lee, KB9ZUV, wrote:

> I currently own a couple of noise bridges for hf work.  I 
> am totally blind, and find them invaluable for setting 
> the tuner.  I am trying to get into vhf antenna 
> construction.  Mostly because the sizes are manageable.
> However, I have no way of checking resonance or swr etc.  
> 
> Is there a vhf noise bridge out there?  Or if not that, 
> does anyone have any other suggestions.  Primarily for 
> 6m, 2m, 1.25m, and 70cm.

There is no reason why an antenna noise bridge should not 
work on VHF or UHF if the bridge is constructed carefully 
with minimum stray reactances and the appropriate values of 
capacitors are used.  The noise output of the zener diode 
drops off rapidly above HF, however, so an amplified noise 
source would be needed.  However, I know of no commercial 
models of noise bridges that would be suitable.  If someone 
were to build his own, some of the NoiseCom diode noise 
sources might be suitable as they produce noise well into 
the microwave region.

With antennas on VHF and above, the use of an antenna tuner 
in the shack is questionable.  High VSWR leads to added 
transmission line losses which are already quite high to 
begin with on these frequencies.  You might easily get the 
VSWR to the rig down to a perfect match, but lose almost 
all your signal in tuner and cable losses.

One common problem with VHF and UHF antennas is that with 
the dimensions involved, a slight change in antenna 
construction can make the resonance outside the ham band.  
Unless you have a general coverage VHF/UHF receiver to look 
for the null in noise bridge output, you may never find 
it.  Note too that an FM receiver cannot be used with a 
noise bridge (although an S-meter might give you a general 
idea of the null if it is extremely sharp - which is 
unlikely on these frequencies).  You must have an amplitude 
sensitive detector.

A good SWR meter is probably your best bet.  It should be 
possible to take the voltage used to drive the meter 
movement and amplify it.  This voltage could then drive a 
voltage controlled oscillator and speaker making "tune by 
ear" a reality.

Long ago, I saw some digital meters with a Braille output.  
They used miniature solenoids to raise pins making the 
Braille characters in a similar fashion to how a dot matrix 
printer operaties.  I do not know if these are made any 
more.

Ball State is not an engineering school, or I would suggest 
contacting the electrical engineering department.  A 
microprocessor based VHF/UHF VSWR meter with a voice or CW 
readout would make an excellent senior design project.  A 
computer science major could probably do the programming 
aspect, but the directional coupler design for the 
measuring end of the instrument would not likely be in 
their training.  However a commercial Bird wattmeter 
could be easily modified (but expensive).

I will be glad to discuss these ideas more off-line if you 
wish.  I admire your desire to experiment with antennas.

        73,  Barry L. Ornitz     WA4VZQ     ornitz@tricon.net