Why does sennheiser hdr120 looses antenna and has poor reception range?

Sennheiser Wireless Headphone HDR 120 Instruction Manual
read

read

You are in somewhat of a fringe area and subject to the vagaries of proper reception. Hopefully you have a good outdoor antenna mounted as high up as possible. You might also consider adding an antenna amp to the downlead to boost up the signal.

Digital signals are not as strong or robust as analog ones and good reception can be thwarted by tree leaves in the path of the signal to your set!!!!!!!!! You should have as clear a line of sight to the transmitter as possible. Relocation of antenna is also another possibility.

The system is working in the 860 Mhz range, so not that common. Most of the time it is cheaper to by a complete new system then to tray to buy components. Just keep looking in the thrift stores you find a transmitter for this headphone for a good price.

Poor reception is ant. No reception is radio. Poor explanation gets poor answer

There are many causes for poor reception on a cb radio one cause may be poor or poorly assembled antenna, a good antenna should be of highest gain and put up as high as possible ,An omnidirectional antenna or Ground plane will hear moderately well
in all directions but a directional antenna or Yagi will hear great only in the direction it is turned you will certainly have to decide what you or your homeowners association will allow.
Also and finally The sun is a major factor in communications Propagation sometimes no mater what your setup the reception is terrible.

check your antenna wire to see if it came unplugged from back of radio

If you have window tinting on the back glass, it will cause poor reception. If there is no tint, check the wiring near the top of the back glass for looseness or corrosion.

location of the antenna is every thing.the other thing to check is you swr.If your swr is to high your reception will be poor

Reception has little to do with grounding. The ground plan is for transmitting. As long as the coax is connected and is in good standing, it should receive fine regardless of grounding.

There are a few reasons you may have bad reception.

1. Turn the RF GAIN all the way up. This is your receive sensitivity.
   
2. Squelch is too high. It should only be turned up enough to cut out the background noise.
3. Poor antenna. If the antenna is cheap, and not made well, this could be the problem.
4. Not enough of the antenna above the vehicle. It needs to be at least 60% of the antenna above the vehicle.

Hope this helps!