Converting a SBGH JED GH6 wideband to a DBGH

[75ohm]

I built a SBGH using JED’s GH6 wideband design using ¼ inch copper tubing. It worked reasonably well in early April, but since the leaves have started to come out, the performance has deteriorated somewhat especially during wet weather. I would like to convert it into a DBGH in order to realize some gain – close to 2.5 dB if I read the threads correctly. My question is; “How much spacing do I use between the bays of the DBGH?”

I see two logical options.
1) Use 211 mm which is the same as delta Z2 for this design
2) Have no spacing. That is the top reflector of the lower bay is the same as the lower reflector of the upper bay. This also has the practical advantage of reducing the total number of reflector pairs to 11 from 12.

The 211 mm spacing would result in phasing lines of 1310 mm assuming that the phasing lines are 25 mm in front of the driven element and are spaced by 51 mm. G0 of this antenna is 110 mm. If I apply 300ohm’s rule of thumb for determining the maximum gain frequency (phasing line length is 2Lambda – 2%), the maximum gain would be realized at 449 MHz or somewhat below channel 14.

No spacing between the bays would result in phasing lines of 1099 mm using the same assumptions as above. The maximum gain would be realized at 535 MHz or in channel 24.

Is there a recommendation for the spacing between the bays? Would using no spacing – that is a common reflector between the upper and lower bay – cause other problems or kill the gain? Or am I on the right track with 211 mm even though the maximum gain is realized at below channel 14? The total height of the DBGH with 211 mm spacing is 2191 mm or a bit over 7 feet.

Thanks for all the great info that these threads contain. I have thoroughly enjoyed this project.

[M.Satterfield]

I am using 1044mm (41") between mine to center it at channel 29 (563MHz) with my "bandwidth of concern" set at Ch 19 (503MHz) to channel 39 (623MHz).

My net gain can be found here:

http://www.wuala.com/m.satterfield/D.../earl-gain.xls

and is ranging from about 15.5 on the low end to over 17.5 on the high end.

I am still playing with phasing wires and also want to play with a shorter distance between the two SGBHs.

HTH.

Mike