Hi Chris. What advantage if any would a short 3-5m vertical have using the CCW High-Z amplifier compared to the same vertical using a 49:1 transformer as described by Lankford? Thanks
Advantages of the High Z antenna amplifier over a 49:1 transformer:
Constant 50 k ohm input impedance over the full frequency range
Common mode choke on the output to isolate the antenna element and ground connection from RF noise coming down the coax from the receiver
The latest version is fully isolated from the antenna and ground connection. This allows it to be safely used in areas that use protective multiple earthing on the mains electrical power system. This isolates the RF ground from being part of the mains ground avoiding any high fault currents that may flow through the station and coax feeder in the event of a fault.
The amplifier is designed so that each amplifier has identical gain and phase characteristics so that multiple antennas can be correctly phased together into phased arrays
The amplifier does give some power gain. The voltage gain is 1 but the current gain is 1000. The RF voltage in the coax is the same as at the antenna element. In a 49:1 transformer thr RF voltage is transformed down b a factor of 7 to match the coax.
In real life use in very low RF noise rural environments the extra gain and common mode isolation of the amplifier allows it to have a lower working noise floor than the 49:1 transformer. In those environments the real off-air noise signal is very low in a short element and would be easily overwhelmed by common mode noise.
This is just a quick review of the advantages off the top of my head. I'll probaly think of a few more once I've sent this post.
Thanks Chris. Just one more question. Does the High-Z amp have any FM filtering? I have a 25kW FM station about 2.5km away on 107.5 MHz. Some amplifiers I've tried get swamped by this. Tom
The High Z Antenna Amplifier uses the common mode choke as a low pass filter to cut off above 30 MHz.
I would also recommend cutting the jumper to the dioide limiter on the output to avoid overload problems before the low pass filter. The diode limiter is only there to protect delicate SDR receivers like the SDRPlay from high RF levels that may damage them.
In your situation I would make sure that the length of the vertical antenna wire is an odd number of quarter waves on 107.5 MHz. This will make the antenna connection a low impedance point to avoid any possible amplifier overload.
For 107.5 MHz 5/4 wavelength is 3.488 m or 11 ft 5 in and 7/4 wavelength is 4.879 m or 16 ft 0 in.
