Dave,

The thing to remember at VHF is that the dimensions of the hand-held
transmitter/receiver box are just as important as the dimensions of the
antenna.

For example, a properly-designed hand-held transceiver plus a rubber-duck,
isolated from ground, is equivalent, in overall length, to a half-wave
dipole.

Likewise, a vertical whip mounted on a vehicle behaves as a halfwave antenna
of length crudely equal to whip length + vehicle depth. (As modified by a
loading coil if any.)

If the rubber-duck (which is mostly a helical loading coil) is replaced by a
rod or wire of longer length, two things happen -

First, the resonant frequency of the system (antenna + box) changes.

Secondly, the antenna pure input resistance changes from its optimum
(designed for) value to an entirely different impedance of resistance +
reactance.

In your case, due to its longer length, the antenna + its new smaller
loading coil (if you fit one) will certainly radiate more efficiently. But
the mismatched transmitter power amplifier (which you havn't got) will now
be appreciably less efficient.

With the rubber-duck the antenna will be matched to the receiver input. But
with a different antenna impedance there will be a considerable mismatch
loss and receiver sensitivity will also be reduced.

Ideally, if the antenna is changed then both Tx and Rx impedance matching
arrangements should be changed. Which you can't do!

So the BEST you can do is change the antenna rod or wire length such that
the overall length of the system (antenna + box) is crudely 1/2-wavelength
at the working frequency and HOPE that you gain more on the swings than you
lose on the roundabouts.

You will likely be more successful on receive than on transmit. But it is
quite possible for performance, especially on transmit, to be worse.

The most likely result is that you will hardly notice any difference. At
best, if you could measure it, you can expect a few dB improvement. Depends
on how big is the rubber-duck you begin with. Don't throw it away. You may
wish you hadn't.

Another thing to remember is that the extremely uncertain hand and arm which
holds the scanner forms an integral part of the radiating/receiving system.
A sweaty palm and an arm held over your head can seriously affect tuning.

Orientation (polarisation) of the antenna relative to balloon antenna
orientation can also make a great difference.

Exact length of antenna (resonance) is a very non-critical matter. Just
connect some wire to the antenna socket and be prepared to prune it.

What does your "Reception is not too great at times" mean. To effect a
noticeable improvement, a 4-times increase in power from the balloon
transmitter would be noticeable. (A 4-times increase in power is equivalent
to twice the height or distance.)

Great fun is available just from all the uncertainty involved.

Happy experimenting!
----
Reg, G4FGQ

===============================

I'm tracking a high altitude balloon on 2m via a handheld scanner, and
in the past have used the standard rubber-duck antenna that the scanner
came with. However, reception isn't too great at times. What kind of
gain increase could I expect to see if I were to replace the current
rubber-duck antenna with a quarter-wave antenna (designed for 2m and
70cm)?

Thanks in advance for any insight!
Dave