Backyard Feed Tests

 

By Tim Alderman

Upset over mis-representation of feed performance, I decided to conduct my own tests.

 

Background

In June 1984 I read a magazine article that changed my life.  It spoke of signals crossing borders, which I had heard as a child with short wave.  Determined to find the best, I went to fifteen TVRO showrooms in my area, and was thrown out of two because I intimidated the salesman with my loud mouth full of technical knowledge.  I finally found Myron Cohen, who was the only dealer willing to come out to my back yard to do a site survey.

 

We became friends while I completed a 5-month after-hours free apprenticeship to learn how to install dishes and feeds.  I kept my “day job” in telecommunications while volunteering to build a satellite dish owner radio talk station at night.  When I lost my day job, Myron hired me to crawl under rich people’s houses.  Every time I get laid off in telecommunications I have a job to fallback to.   I also taught feedhorns in a seminar to eager students in Auckland, New Zealand in 1996.

 

Myron and I always had differences of opinion when it came to the feedhorn.  I was a firm believer in the National ADL brand, while Myron used “Brand C”, which had 80% of the market.    

 

The RP1 is a designation for the moded feed type which gives a slight (about .2dB) improvement over the more common scalar ring design used by all other feedhorn companies except Miralite. Gerry Blachley, the owner of National ADL, told me he was forced to introduce the RP3 scalar ring to make his feeds look more like the competition.   The RP2 he also was forced, he said, to introduce to meet the “gold ring” deep dish designs then becoming so popular.  The RP1 was, and still is, a shallow dish feed with superior sidelobe performance, all other things being equal.

 

Gerry sold National ADL.  The name was changed to Patriot ADL and moved back east.

 

Feedhorn Disaster Strikes

This past fall, after many turns and twists in C band TVRO, Myron once again hired me, I was once again out of a job.  He decided to try my brand and he bought a Patriot ADL RP1-OR100 feed. I have been using an original National ADL on my dish for over a decade. I tried this new Patriot feed on a friend’s dish with disastrous results. At first I blamed myself for this when I discovered errors in my workmanship.

I next took that Patriot feed to a customer site, thinking I would get the same performance as I had enjoyed with my National.  After all, they were supposed to be the best feeds with dual LNB’s and the highest cross-pole isolation available in TVRO, for simultaneous dual polarity usage.   Five hours on the job site later, I gave up and went and got a “Brand C” feed from Myron’s boneyard.  The results made me eat crow when the dish performed like gangbusters.  I decided to take a day off and do some tests.

 

 

 

Analog test results

 

Using my own backyard dish, on satellite C4, I discovered there were two opposite polarity in-the-clear shopping channels on transponders 9 and 10.  C4 also has 4DTV digital signals, including my new favorite, Discovery HD theater. 

 

I removed my National ADL RP1-OR100

and swapped in the feed which has suspect. Immediately I noticed a substantial difference in polarity.  The Patriot RP1-OR100 would not properly cross-pole null polarity.  As I was testing, with a TV set to monitor analog on one pole and my spectrum analyzer to observe the other, I had, at one point, removed power to one LNB.  Immediately the other LNB acted differently.  I could polarize the feed normally.  Checking further, I found that applying power to the second LNB caused interference to the first.  The cross pole null rotated about 45 degrees and it lost about 3dB in signal.  This caused sparklies on all transponders.

 

I know that MSE was the last LNB manufacturer to use input isolators, and that they went under about a decade ago.  However I have a pair so I tried them and found that indeed the one Norsat Digital LNB was causing interference with the other. I measured the loss of polarization at an astounding 17dB with 3dB additional insertion loss.  There are no LNB’s being manufactured today for TVRO with isolators.

 

Having discovered the source of my problem, interference, I went onto test yet another brand new Patriot ADL RP1-OR100, this one lacked a center tube, but was still basically the same indirect waveguide design.   It had slightly less insertion loss but still no where the benchmark performance of the National ADL with the same model number.  It also exhibited interference, but again, was less.

 

Next I decided to expand my tests and try both historical and modern feeds. All feeds are C band only, and all are dual polarity, except the “brand C” superfeed.  The Patriot ADL RP3-OR100 was almost as good as the National RP1-OR100, but still exhibited slight, very slight interference as well as had 1dB more insertion loss.  

 

I next dug out of my boneyard an original “Brand C” superfeed, which is single polarity and has no scalar adjustment. The mouth of this feed is set for a .4F/D ratio dish as it sets in flush with the outer rings, definitely a shallow dish feed.   As expected, it was slightly better in cross pole isolation than my reference National ADL feed.

 

Finally, I found a Miralite “Miracle Feed” dual polarity feedhorn and decided to use it with scalar rings.  The Miralite actually uses a moded feed like the ADL RP2, which is deep dish territory and not in my back yard.  However it also fits in the Patriot ADL RP3 scalar. It’s performance was very nearly equal to the National ADL RP1-OR100 and perhaps very slightly better. As it’s throat is adjustable, I was able to confirm that 1/8” past the scalar ring was the proper place.  To far in and it started to lose gain. To far out and it was picking up over-illumination noise.

 

These tests were done with analog signals as well as observing a SCPC digital carrier which made a nice flat topped signal for cross pole and insertion loss measurements.

 

Digital Tests

4DTV has several feeds on C4.  I used “TV Game show” which is a horse racing channel. Something nice and stable but which I would normally never watch.  The receiver allows me to check both signal level and distortion level by way of a digital error rate indicator.  The higher the number, the better the signal strength for the first number, and lower distortion for the second. 100 is deemed “perfect” arbitrary figure. (all pictures clear except where noted)

 

Power applied to 1 LNB only:

Brand C        National ADL     Patriot ADL         Patriot ADL       Miralite

Superfeed     PR1-OR100      RP1-OR100        PR3-OR100      Miracle feed w/scalar

96                      96                     95                            95                   96

95                      92                     51 (picture tiling)      86                 95

 

Power applied to both LNB’s simultaneously:

N/A                  NOT TESTED       96                            95               NOT TESTED

        1  (black screen)      85

 

Conclusion

 

Both analog tests and digital arbitrary numbers indicate that Patriot has compromised the National ADL design.  Range tests need to be done to confirm this and provide a more exact set of numbers.  However, with my feedhorn experience, coupled with today’s lack of isolated LNB technology available to TVRO, I would say that the edge formerly enjoyed by National ADL when Gerry Blachley owned the company has not been retained by Patriot.

 

There are other factors, such as price and the more powerful servo motor that used to be in the National ADL single LNB feeds, may affect your C band TVRO feedhorn purchase decision.