Sunday, May 27, 2018

An Off-Center-Feed Antenna for 160 through 10 meters

It's been a long time since my last post. Life and laziness intervened, mostly the latter.

This antenna is not for small spaces. It is nearly 250 ft long but is you can manage to negotiate successfully with neighbors or have the space I believe you'll find it's worth the effort.

I have spent some effort on this particular antenna researching the commercial market and test my homebrew version that is shown here.

Commercial antenna's tend to use matching networks (baluns) and coax feed systems so I started out trying that arrangement on mine.

All the antenna wire used on this antenna is #16 AWG copper clad steel from Davis RF tuned to the low end of 160 meters. Overall length was 243 feet at 30 feet above ground. The feed point was set at 59.5 feet.

All feed line arrangement were first test with my MFJ-259b antenna analyzer and the data for each band was recorded for future reference.

First feed line arrangement - 100 ft RG8x with a 1:1 balun. On air performance was acceptable on 160 and 80 but poor at best on all other bands. The on the air performance agreed with the analyzer's measurements. A very poor match, > 5:1*, above 3.7 MHz.  I operated this antenna Sept 2017 through Dec 2017.

Second feed line arrangement - I connected a 9:1 balun at the feed point and connected the same RG8x feed line. The analyzer measurements were told the story. It was apparent that the feed line loss due to mismatch was little better at 40 meters and up. much worse on 160 and 80 meters. No surprise here. I didn't even put the antenna on the air.

Third feed line arrangement - 78 ft of 450 Ohm ladder line from the feed point to the 9:1 balun just outside the shack, then 20 feet of RG213 into the shack to the radio.

This arrangement has performed well beyond my expectation 160 through 10 meters since January 2018. I've made over 2000 QSO world wide on FT8, CW, and SSB combined from Jan 2018 through May 2018. I also ran WSPR for many hours in band hopping mode both transmitting and receiving. I ran some spot checks using the CW Reverse Beacon Network (RBN) using my MFJ-464 CW KEY/READER  in beacon mode by sending "TEST TEST DE K7PDW". The results were very acceptable with worldwide reports for both WSPR and RBN.

SWR readings on my IC-756PROII are as indicated below. The internal tuner will handle all the bands except 80 and 40 on the high end.

160 - 2:1 or less across the band
80 - 3:1  or less below 3.6MHz, 5:1 or higher above 3.7MHz
40 - 3:1 or less below 7.06MHz, 5:1 or higher above 7.15MHz
30 - 1:1 all the band
20 - 1:5 or lower for all the band
17 - 1:1 all the band
15 - 3:1 for all the band
12 - 3:1 for all the band
10 - 2:1 or lower  below 29.2MHz
Not tested on 6 meters

I've had a 67ft doublet with 87 ft of 450 Ohm ladder line to a 9:1 balun with 20 ft of RG213 into the shack for years that I used for 40 through 6 meters. I have taken it down because this antenna out performs it so well. I now have an Arrow 52-4s four element beam on 6m.

If you have the space to erect this antenna, I recommend you give it a try. It's not very expensive to build, the balun being the highest price item on my system. I use a DXEngineering DXE-BAL450-H10-A balun (pricy). It is rated at 5kW on CW and 10kW on SSB. If you don't run over 100W then a less expensive one will work for you.