I'm trying to receive a UHF TV signal 86 miles away. Considering the height of the transmitting antenna and the height of the receiving antenna, I should be able to just make line-of-sight between them... but the signal will be weak and probably perturbed in a number of ways.

I can now occasionally receive this signal using 1 DB8 "bedspring" antenna. The popular DB8 TV antenna consists of 4 vertically stacked bowties in parallel with 4 vertically stacked bowties. In the center of the lines that connect these two arrays in parallel is a matching transformer to convert the 150 ohm balanced signal from the parallel arrays to a 75 ohm unbalanced output.

I want to vertically stack two DB8s to try to achieve a theoretical +3db additional gain. It seems I should be able to do this by removing the matching transformer from each DB8 and connecting the two antennas in parallel by simply running two parallel lines (spaced and sized appropriately) from the center feed point of the top DB8 to the center feed point of the bottom DB8... in place of the matching transformers. This puts the 150 ohms of the top DB8 in parallel with 150 ohms of the bottom DB8 to yield 75 ohms at the center of the lines connecting the two antennas. Then, using a 1:1 current balun, I can convert the 75 ohms balanced to 75 ohm unbalanced to match my 75 ohm coax downfeed. This is the lowest possible loss method I can imagine to combine two DB8s.

Sounds easy but I've been around a long time and in my experience, antennas seldom behave exactly the way theory predicts. Has anyone ever tried this? Is there a better way?

(Edit: Following Glenn's excellent advice, I've disassembled the baluns on each array and hope to begin experimenting with the phasing harness this weekend. For those interested, below are pictures of the PC board inside balun enclosure.)

Balun - LC side Balun - Ground plane side

  • $\begingroup$ Spacing is important. You can also gain 3db by adding a reflector plane. $\endgroup$
    – user10489
    Sep 26 '18 at 11:40
  • $\begingroup$ Looks like the DB8 already has reflectors. $\endgroup$ Sep 26 '18 at 15:37
  • $\begingroup$ Are you sure the transformer is 2:1? (150 ohms to 75 ohms) I don't see it mentioned in the specs for the antenna. $\endgroup$
    – Brian K1LI
    Sep 27 '18 at 19:38
  • $\begingroup$ To Brian's point, I am not sure. The "transformer" now used is some kind of asymetrical (meaning it's different on the side connected to the left array from the side connected to the right array) PC transmission line, the output of which is unbalanced and connected to the 75 ohm coax. My assumption of 150 ohms is based on the manufacturer's past practice of using conventional 300 ohm to 75 ohm baluns at the center feedpoint of each of 2 4-bowtie stacks, then combining the 75 ohm feeds from those two baluns in a conventional combiner, resulting in a single 75 ohm downfeed. $\endgroup$
    – akatengu
    Sep 27 '18 at 22:09
  • $\begingroup$ This 2-balun design was later changed. The new design used a transmission line to connect in parallel the side-by-side arrays. The center of that line connects to the asymetrical PC transmission line I mentioned above, the output of which is unbalanced 75 ohms. Of course, all of this assumes the designers in each case sought impedance matching throughout the system to arrive at the 75 ohm output impedance. $\endgroup$
    – akatengu
    Sep 27 '18 at 22:14

The degree of additional gain from phasing two of the arrays will largely come down to the phasing harness and the balun.

Since you are focusing on a single UHF frequency, I recommend an open line phasing harness between the two arrays after removing the supplied balun on each array. Use multiples of 1/2 wavelength of open wire transmission line from each panel. This will replicate the impedance of each panel and result combined impedance of Z/2. The characteristic impedance of the phasing harness sections does not matter if you use multiples of 1/2 wavelength. Close attention to the length of each line is important in order to ensure proper phasing from each panel. Don't forget to adjust for the velocity factor of the phasing sections. If completely open lines with no insulators are possible, you could achieve a velocity factor close to 1. If you have a few spacers, the velocity factor may approach 0.98 or so. Multiply your calculated 1/2 wavelength multiple by the velocity factor to determine the physical length required.

If you have an antenna analyzer or VNA at your disposal, it would be helpful to measure the complex impedance of each array and then confirm the resulting complex impedance of the phased arrays. This will remove a great deal of the ambiguity in the solution.

In order to transform the combined impedance to an impedance more suitable for a balun, you may have success with a shorted 1/2 wavelength (or longer) open wire transmission line of any impedance. Connect one end of the open wire transmission line to the junction of your phasing harness and temporarily short the other end. The balun is connected between these two points. By adjusting the connection point of the balun and the position of the shorting element, you can generally obtain a suitable match for the balun. Here again the use of an antenna analyzer or VNA would be helpful in reducing the iterations necessary for the best match.

If you are up for home brewing an antenna, you may have success with a trough reflector design. With a 45° vertex, a theoretical gain of 17 dBi can be achieved with a single dipole element. At UHF frequencies, the reflector can be made of a screen mesh. The ARRL Antenna book gives the basic design parameters.

  • $\begingroup$ Excellent, as usual. :-) What you describe used to be popular in the '50s and '60s for both TV and amateur UHF use. $\endgroup$
    – Mike Waters
    Sep 28 '18 at 19:05
  • $\begingroup$ Thank you, Glenn. I'll begin experimenting with the phasing harness this weekend. It didn't occur to me that the impedance of the phasing lines was a non-issue at multiples of 1/2 wavelength. I plan to use aluminum clothesline wire which seems to be almost identical to the wire used by the manufacturer to connect the bowties. I'm going to try to wind my own bifilar 1:1 transformer first. That seems easier to me than trying to use a shorted transmission line 'cause I don't have the right stuff to mount it close to the feed point. Might have to come back to this, though. $\endgroup$
    – akatengu
    Sep 29 '18 at 18:26

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.