View Single Post
Old 11-Jul-2013, 7:03 PM   #40
Senior Member
Join Date: Jan 2010
Location: Dallas, TX
Posts: 129
Half Wave Coaxial Baluns for VHF

Recalling the VHF antenna project objective:

To receive WHAS-11 and WBNA-8 located in Louisville KY.

TVfool report (both stations listed as 2-Edge):

		Noise Margin	Distance (miles)

WBNA-8           -7.2 dB	79.3
WHAS-11		-14.9 dB	101.1
Observations over several weeks with the stacked YA-1713 antennas on the main tower, it was determined that an additional gain of 7 dB may be required for reliable reception of WHAS-11.

After testing for a few weeks at the test location (garage) with some high-gain antennas, it seemed like there were few options that could provide 7 dB gain above that obtained with a pair of YA-1713 antennas. A stack-of-two large, either Wade/Jerrold or Channel Master antennas seemed likely to be insufficient, and a quantity of four of such antennas was unavailable.

At about half the length and 1/3 the width, the gain of the channel-cut antenna tested to be within about 2 dB of a single large antenna.

Since it was desired to receive two stations separated in frequency, WHAS-11 and WBNA-8, then using channel-cut antennas for a single channel could be problematic

Fortunately, in this case, a characteristic of Yagi antennas is that gain decreases less rapidly on the low side of the design frequency. Since the design frequency would be weak signal channel (channel 11) and the lower in frequency, channel 8 signal was considerably stronger (not requiring as much gain), then a channel-cut antenna might suffice.

Considering the above information, it seemed a pair of long channel-cut Yagis designed for channel 11 might meet requirements. Since a channel-cut Yagi of about 85 inches in length delivered a gain within ~2dB of a VIP-306 or a CM-3610, then a channel-cut Yagi of 170 inches probably could exceed the performance of either of the larger antennas.

Location: Dallas

In an attempt to maximize signal from such antennas, half-wave coaxial loop baluns were designed.

A small channel-cut Yagi for channel (Wade 5y10s) 10 was purchased for balun design and testing.

At that time, there were 3 VHF stations (Channels 8,9, and 11) in Dallas. The signal paths were line-of-sight and the signals were strong and steady at the Dallas test location. The broadcast DTV signals provided signals for gain comparisons between antenna modifications and balun types.

Several loop baluns were constructed and reception compared to each other, and to commercial baluns (using the small Yagi and broadcast DTV signals).

After suitable half-wave loop balun designs had been constructed. The input impedances of several balun and antenna combinations were quantified. Knowing complex input impedance allows SWR to be calculated. The impedance measurements were performed using a vintage Boonton/HP RX250A The RX250A is capable of accurately measuring complex impedance from 500 kHz to 250 MHz. Measurements confirmed that the three-bar folded dipole had sufficient bandwidth and that a half-wave coaxial balun provided a low SWR match to 75 Ohms.

Half wave baluns were constructed using both RG6 and RG-62 (93 Ohms). In agreement with theory, the RG-62 cable provided the widest bandwidth and best match over channel 8-11 and thus the lowest overall signal loss.

Note: At, or near, the loop half wavelength frequency, loss was less with RG-6 cable. Away from that frequency, losses were less with RG-62.

Attached Images
File Type: jpg 5y10s loop balun.jpg (98.7 KB, 7191 views)

Last edited by tripelo; 13-Jul-2013 at 3:36 PM. Reason: Add note
tripelo is offline   Reply With Quote