[Elecraft] low-noise 40m RX antenna

[Gerald Pfaff]

Wayne Burdick, N6KR wrote:

I think this topic might be of general interest for weak-signal DX and QRP =
work on 40, but you seldom hear any discussion about low-noise antennas =
outside of their application to 160 meters.

There are several commercial made =93magnetic loops=94 for 40m. I am using =
the MFJ model. The big advantage is the selectivity. It acts like a =
=93big=94 preselector. The disadvantage is that you have to re-tune every =
few kHz. Once you get the hang on it it works really fast. The good thing =
about it, you do not need a separate TX-antenna. K2 # 1185 with KPA100 and =
the magnetic loop work just fine. (HB9 to W1 =96 W4 QSO=92s on 40m. Well, =
those W-stations where not exactly QRP in the WAE-Contest ;-) Most of the =
credit for these contacts are due to their antennas.
There is a simple receiving antenna; too. It is a loop made out of RG213 =
coax cable. The shield is split on top of the antenna. At the bottom both =
split shields are connected to the shield of the coax to the RX. The inner =
wires connects to a capacitor at the bottom of the loop. One side is =
directly connected to the inner wire of the coax to the RX. Since you do =
not transmit just peak the signal and do not worry about SWR. As I recall =
it is described in an ARRL Antenna Handbook. Construction is cheap. Just =
some feet of coax, a variable C and a supporting structure.=20
This antenna gives equal characteristics like a magnetic loop. But you need =
to peak it for the frequency of interest. If this antenna is out of reach =
you need to tune it remotely. I used this antenna just on the desk in the =
shack with good results.
The signal strength may not be what you expect with a full size dipole. But =
QRM is reduced on this antenna. And when used standing on one side you can =
null out, or reduce, unwanted signals.
Laying the antenna =93flat=94 should result in an omnidirectional pattern. =
This could match the pattern of a Groundplane / Vertical antenna for =
transmitting. I did not try this.

73 de Gerald, HB9iRF      =20







73 de Gerald,  HB9iRF=