Active antenna parallel to main antenna in V/U Handheld?

Hello,
I have a handheld transceiver that transmits on V/U and receives wideband.
This is my idea in connecting an HF+50MHz active antenna in parallel, whereas at the same time be able to transmit from the main antenna at V/U, without the HF active antenna to be affected.

What do you think of my idea?
I wirry about the parallel connection of the antennas, what happens to the impedance at this point and will this affect reception and transmission much?

We have to look at the antenna geometry. If they are close, both functions will be probably affected.
- the diode clamp of the receive antenna will partly short the transmitted signal and even cause unwanted hamonics.
- the transmitter antenna could reduce the field strength in it's vicinity.

FvM said:

We have to look at the antenna geometry. If they are close, both functions will be probably affected.
- the diode clamp of the receive antenna will partly short the transmitted signal and even cause unwanted hamonics.
- the transmitter antenna could reduce the field strength in it's vicinity.

Click to expand...

The best way to describe the dual antenna is like a handheld transceiver "rubber duck" antenna.. The transmitting antenna is just a plain V/U rubber duck. However the receiving active antenna, is capacitive and it is a metal tube just below the body of rubber duck antenna.
The two antenna parts are connected together but not electrically, just mechanically, to form a single antenna that is composed of two.
The only electrical connection of the two antennas, is at the connector side.

Let me explain a bit of how the idea works:

On RX
The RX antenna receives and the input clamp ensures amplifier protection in case of extreme nearby signals.
The output signal passes through the output clamp, to ensure protection of the receiver (pga103+ can output a big level)
The output signal passes through the LPF, which has no significant effect below 60MHz.

On TX
The signal goes to the TX antenna and transmitted.
The LPF ensures no significant signal will pass through to the preamplifier, since the TX frequencies are 144 and 433MHz (or close) and the LPF starts to cut-off from 60MHz and up. Simulation shows a worst case attenuation of 30dB, which will allow 5mW to pass throug the filter to the output of the preamp, assuming a max input power of 5W.
This 5mW signal may be further clamped by the preamp output clamp.

If the preamp is powered on during transmitting, The ferrite bead and the input clamp will help somehow reducing the input signal to the preamplifier from the transmitting antenna, but not eliminate it entirely.

There is an idea I have of powering the preamp off during transmitting using a mosfet switch, but it might introduce harmonics and other spurious. A miniature relay might be a solution. In that case an LPF might not be needed.

As said, I do not what happens at the connector side, with my idea as far as concern impedance and if this is going to affect too much the RX/TX operation in practice (high VSWR on transmitting, or signal attenuation on receiving due to antenna-transceiver impedance mismatch)

FvM said:

We have to look at the antenna geometry. If they are close, both functions will be probably affected.
- the diode clamp of the receive antenna will partly short the transmitted signal and even cause unwanted hamonics.
- the transmitter antenna could reduce the field strength in it's vicinity.

Click to expand...

HAve you got any comments on my points above?
Thank you