The
Channel Master 8-bay coupled through a CM-7777 and a Join-tenna provided reliable reception of WCTE-22 (~ 65 miles distant in Tennessee).
It was noticed that Channel 23 WPXK, also in Tennessee, might also be receivable. WPXK-23 is separated by about 54 degrees in azimuth from WCTE-22. If both stations were receivable, they could probably be accommodated by the single existing Join-tenna.
In addition, a third station Tennessee station, WBXX-20 located in azimuth between WCTE-22 and WPXK-23. If channels 22 and 23 were receivable, then due to higher signal strength of WBXX-20, it might be pushed through the same Join-tenna.
At distances of 65-75 miles, the TVfool noise margin levels and azimuth angles were:
Code:
Noise Margin Azimuth (Deg.)
WPXK-23 -0.4 dB 145
WBXX-20 +13.2 dB 152
WCTE-22 -2.0 dB 199
Note: WBXX-20, in azimuth, lies between the weaker stations & closer in angle to WPXK-23.
A wide-beamwidth antenna with moderate gain could be a solution. Some candidates:
· Channel Master CM-4221
· Antennas Direct ClearStream C2
· Custom corner reflector (CR)
Another option could have been something like an Antennas Direct ClearStream C4 or Winegard HD-8800 with:
· Relative phase of right–left (RL) feed lines reversed
· RL Bay spacing optimized for split-beam lobes at 54 degrees separation
Some features of a custom CR antenna were appealing, one of which was it could be optimized for narrow range of channels around channel 22.
First, a CR backbone was retrieved from a scrap Antennacraft all-channel antenna. Then reflector elements were scrounged. Shorter reflector elements can yield wide beamwidth, but compromise some gain. For a few reasons, a moderately short reflector length of 16” was selected. The driven element was made from longer trimmed-to-length elements and supported by recycled insulators.
When the prototype was assembled, it was simulated in a computer model using
Arie Voor’s 4NEC2 software. One objective was to get every tenth of dB gain while maintaining a good match to 300 Ohms. Since wide-beamwidth antennas usually have only moderate gain, then in this case it was important to have a good impedance match to optimize efficiency.
The prototype was tested with three preamps:
· CM-7777 with low loss balun
· CM-0264 (internal balun)
· Blonder Tongue Vaulter III (internal balun)
The image below shows the prototype and a couple of preamps on the test mast.
The main variables in optimization with 4NEC2 software were:
· Driven element length,
· Position of driven element relative to apex
· Position of reflector elements near apex
After a few rounds of optimization; computer simulations, trimming, and on-the-mast testing, the CM-0264 was chosen as the preamplifier. The final tweak on the main tower was trimming the transmission line between the CR and the preamplifier for maximum signal.
The image below shows the installed corner reflector on the main tower. Most obvious change in the two photos is the position of the two reflector elements near the apex.
With antenna direction at a compromise between the two weaker signals, the signal strength was not high on either of them. Fortunately, for all three signals being 1-Edge diffraction, their signal strengths were relatively stable, especially considering the distance of 65-75 miles. In normal viewing, no TV image dropouts have been seen or reported. Image dropouts have been seen when severe thunderstorms were between the stations and the receiving location.
A CM-0264 preamplifier was modified to replace its internal balun with an internal half-wave coaxial loop balun. In tests, the modified CM-0264 yielded ~1dB more S/N margin for all three stations. It has not been installed.
Early in 2011, WCTE-22 increased power to 200 kW (from 57 kW). This power increase allowed a re-positioning of the antenna in azimuth, with improved signal strength levels for all three stations.
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