DXM-90-40 - - - 90c long 40m Mobile Antenna
HOW GOOD CAN SUCH A SHORT WHIP BE?
Although I already own a normal single band 40m helical whip antenna, I purchased this short stubby antenna for convenience.
My VW Caddy (not Cadillac, rather VW Mini Van "Caddy") is about 2m (6' 6") high. Our maximum permissible height for antennas in DL is 4m from ground to tip at the top. I am unable to drive with the old whip installed on the roof.
The DXM-40 is just 90cm long so my overall height with this antenna installed is just under 3m.
So how good is it?
Good compared to what?
Well first off we will compare it to a normal whip - an MFJ-1640T, 40m monoband whip.
The MFJ-1640T is about 7 ft. 2.13m tall. With this antenna my car is 4.13m high at its top - about 5 inches longer than permitted by law.
In order to get an accurate view of the feedpoint impedance, I removed the coax from the magnetic mount and inserted a T-Coupler between the antenna and the mount. Then I connected the RigExpert AA-54 directly to the T-Coupler.
THUS, the results shown below are with ZERO COAX.
Surprisingly, the impedance came in at about 30 Ohms.
Obviously this is only the feedpoint impedance, not the radiation resistance.
An antenna this short must surely be around 5 Ohms of radiation resistance.
I don't know what is inside of this antenna but whatever it is, it seems to provide a fairly good match.
Once I replace the mag mount with a proper mount and run a short 1m long piece of coax inside of the car, I will measure again and then match with an L/C network.
To be continued.