Daytime Testing with an Active Whip
The antenna used for this was an LF Engineering H-900 active antenna, mounted approximately 30 feet up on my
roof. I did not switch on antenna's preamplifier (which produces too many mixing products and images), and the
output was fed directly to the input of the splitter.
Using weak signals on 550, 920, 1330 and 1550 khz, each with steady daytime signal strengths, the direct-coupled
receivers experienced severe images and/or overloads from local stations. I live within 15 miles of several 10-50
kilowatt stations, and their daytime patterns are such that mixing products can be found at various places all over
the dial. While there were parts of the band which were relatively free of these effects, much of the band was
rendered unusable. Interestingly, on problem frequencies the indicated signal strength was steady at 50 dB, with 0
dB signal-noise ratio; it is possible that the receivers became desensitized by the massive input.
With the passive couplers, similar problems resulted if the coupler were touching the receiver. However, by holding
the couplers an inch or two away from the receiver, the problem was cured, and the following were the results:
• The 12-turn flat ferrite bar coupler produced the best results, with slightly better signal strength and signal-
noise ration than the 8-turn air core coupler. The 8-turn air-core coupler actually worked best here with the
receiver outside, next to the coil.
• The 20-ferrite brick had the same performance as the 12-turn bar, and the custom-wound air core coupler
had the same performance as the 8-turn 4x5” coupler. As a result, these two couplers were not used in later
tests, since the ferrite brick was cumbersome to use, while the custom-wound coupler made the controls and
LCD screen on the receiver inaccessible.
• The C Crane coupler pulled up the rear, with signal strengths down approximately 5 dB compared to the
other passive couplers.
To see if the receivers being used were part of the mixing product/image problem, I also tried an Tecsun R1212a
analog receiver, and the results were the same: massive mixing products and images when directly placed in/next to
the couplers, and satisfactory results with an inch or two of separation. Therefore, it appears that portable are often
not designed to accept such large broadband antenna signals. Connecting the antenna feed to an Icom R75
communications receiver yielded only the occasional mixing product, normal for daytime conditions here.
Daytime Testing with a Super Loop
I have a 9x10 foot “Super Loop”, corner-fed with the new Wellbrook FLG-100 antenna head. The antenna is
pointed north, generally away from most of the high-power stations near my home. Unfortunately, there are two
local stations within 5 miles of me which nevertheless provide ample RF input, such that they mix with the other )
somewhat attenuated) stations up and down the dial.
As a result, coupling the Super Loop to the receiver produced essentially the same results as with the active whip:
the directly-coupled receivers were severely overloaded with mixing products and images, while the passive
couplers had similar performance relative to each other as with the whip if separated from the receiver an inch or
two. The C Crane coupler, the weakest of the passive couplers, was able to be held directly against the receivers on
most frequencies.
To see if lowering the input to the receiver would help, I fed the Wellbrook antenna output to a Quantum Phaser,
which has a continuously variable RF gain control. This enabled all of the passive couplers to work well, in that
backing off of the gain to certain point resulted in clean reception. I noted that, because of the unique AGC
circuitry on the Tecsun radios, if I started out with too much gain, I had to cut the gain all the way to near zero and
then gradually increase it until the right level was attained; otherwise, without this “reset” of sorts, the receiver
would not readjust if coming down from strong signal levels. Unfortunately, even by carefully adjusting the gain, I
could not preclude images and mixing products from swamping the directly-coupled receivers.