[werk_ag]

...with a 164mm lead connected directly to the SMA connector on the Tx unit. I guess the lead was cheaper - although buying the SMA connector from Jaycar was more expensive than the antenna from Ebay 

As said, avoid using any lead cable on the TX unit. Attach the antenna directly to the SMA connector, alternatively try the uncle_bob suggestion, but connected the solid wire directly to the central pin on the TX pcb. When the use of a lead cable is absolutely necessary, its length should be calculated to be exactly a multiple of 1/4 of the wave-length (to avoid high SWR - which cause that TX power is returned to transmitter instead of irradiated, risking damaging it), this is one of the the reasons why those antennas with a cable are the poor performers specially with low power transmitters.

The SMA connectors for the pcb cost arround 1€ on ebay.
Here an example:
https://www.ebay.com/itm/SMA-female-pin-...SwcnpTmy8a

The Rx unit was located on top of the windows server, moving the Rx unit about 30cm away from the server has bought the efficiency up to 66%. When the new antennas arrive will see what they do.

I'm not surprised, the PC's (as TV's) are a strong source of interference for radio receivers.