The main problem with it is that the two stages being connected with the
wire will no longer see the same impedances they see when operating.
This can cause a multitude of changes in their operation, including
modifying their frequency responses and gains. It can also cause
instability that's not present when the intermediate stage is in place.
The amount of the changes depends greatly on the nature of the cicuits
being connected, as well as the frequency. The changes will almost
certainly be severe at UHF and considerable at VHF. At HF, you might get
by with it and you might not, depending on the circuits.

It's good practice to design each stage for a standard input and output
impedance, with 50 ohms being overwhelmingly the most common choice for
RF circuitry. That is, each stage should be designed for correct
operation when driven from a 50 ohm source and terminated with a 50 ohm
load. The great benefit to this approach is that it allows independent
testing and characterization of each stage with a standard set of test
equipment. A secondary benefit is that any stage can be replaced with a
jumper of 50 ohm coax without changing the impedances seen by the
connected stages.

Roy Lewallen, W7EL

Paul Burridge wrote:
Hi,

Let's say I'm trouble-shooting a circuit board with several stages of
RF amplification in addition to a primary source in the form of a
VCXO. I'd like to be able to bypass certain parts of the signal path
by the use of a jumper wire. This wire may need to be up to 4 inches
in length to have sufficient reach between stages. Can anyone see a
problem with this? I mean, rather than a single wire, should I use
some sort of grounded outer lead like coax to carry the signal?

at HF
at VHF
at UHF