[NLRS] [BC'ers] Give away + Need advice on DirecTV dishes

[Dr. Gerald N. Johnson]

Gain ia almost always inverse to directivity or beam width. His LP gain 
is 6 dBi, his higher frequency Vivaldi is about 10 dBi, the dual mode 
feed for an offset dish he checked for me separately last year at CSVHF 
was about 12 dBi. It was optimized for satellite reception on 11 to 12 
GHz tested at 10.4 GHz.

The f/d of the 18" offset dish is about 0.7 while most commercial prime 
focus dishes is often .4 or lower so the feed for the offset dish needs 
to have a considerably narrower pattern for efficiency and rejection of 
noise sources than that for the prime focus dish. Indeed many a prime 
focus feed needs to have nearly 180 degree beamwidth to make good use of 
the dish, while the feed for the offset dish needs to be more like 50 
degrees at the 10 dB down points. There are some comparisons and some 
dual band feed designs in Paul Wade's (W1GHZ) antenna book on line 
chapter 6_9 part 2. The file name is chap6_9p2.pdf. He has a lot more on 
single band feeds.

It is wrong to say it won't work, but it won't work as good as proper 
single band feeds at this stage of development. The compromise will 
probably cost 6 to 10 dB gain under the proper feed and on bands where 
10 dB means 100 km range that hurts pretty bad.

Both Vivaldi and LP can be designed for higher gain though 10 or 12 dBi 
makes for a long LP and then the phase center moves along the boom a 
great deal with frequency change which is another serious problem 
causing this type of multiband feed to only be at the focal point for 
one band.

73, Jerry, K0CQ

On 6/7/2013 8:55 AM, David Palm wrote:
> Jerry wrote:
>
> The Vivaldi that Kent sells is 10-25 GHz, not very useful on 3.4 GHz. His
> PCboard log periodic that does cover 2.1 to 11 GHz has less gain and I
> think the spill over the sides of the dish will be great, hurting the
> transmit gain and the receive signal to noise ratio.
>
> The Vivaldi that I got from Kent is 5 - 18 GHz, but he says it works down
> to 3.5 GHz.  It's quite a bit bigger than the 10 - 25 GHZ model.
>
> So Jerry, I wonder if you can clarify: are you saying that the Vivaldi will
> do a better job at limiting overspill than the log periodic?  Or are they
> both equally bad in that regard?
>
> Donn wrote:
>
> I look forward to seeing the multi-band dish on the antenna range.
>   (Assuming the WX is such that we can do another antenna range.  We got
> skunked at Aurora last month and next week's make up is iffy with the WX
> pattern we seem to be in.  Maybe later in the summer....
>
> Thanks for your encouragement, Donn.  Yes, if the antenna range is offered
> later in the summer I'm going to try hard to get up there for that.  The
> earlier dates have not worked for me, but I'd love to have another crack at
> it.
>
> 73,
>
> David  W9HQ
>
>
>
> On Fri, Jun 7, 2013 at 12:23 AM, Dr. Gerald N. Johnson
> <geraldj at netins.net>wrote:
>
>> The Vivaldi that Kent sells is 10-25 GHz, not very useful on 3.4 GHz. His
>> PCboard log periodic that does cover 2.1 to 11 GHz has less gain and I
>> think the spill over the sides of the dish will be great, hurting the
>> transmit gain and the receive signal to noise ratio. The dish will
>> concentrate the received signal on the feed, but the feed will see noise
>> sources like hot earth from the spillage.
>>
>> You could make a Vivaldi for a lower frequency range, just make it 3 times
>> the size of his 10-25 GHz and it has better gain than the LP, but still may
>> spray RF past the edges of the dish and pick up some noise sidelobes. It is
>> possible to array Vivaldi's but it gets messy.
>>
>> 73, Jerry, K0CQ
>>
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