[NLRS] [BC'ers] Fwd: Re: Anybody have experience with multi-band feeds?

[Dr. Gerald N. Johnson]

IF you are getting the gain, yes, but as microwaves go the dish hasn't 
much down at 2.3 according to Donn's table.
> 10GHz       28-29dBi
> 5.7GHz      24-25
> 3.4GHz      20-21
> 2.3GHz      14-15

Comparing those to the graph on page 70 of the International Microwave 
handbook its pessimistic, but probably not far off for the Vivaldi feed. 
Maybe even optimistic for the Vivaldi feed on an offset dish where the 
F/D is way different than optimum for the Vivaldi feed.

I didn't quickly find a nice table of looper gain vs band vs lengths so 
will have to guess at data.

At 1296, N6CA says 6' of boom gets 18 dBi and 12' of boom gets 20.5 dBi.

So about 3' of boom might get almost 18 dBi at 2304, 2.4 feet would give 
that at 3456, or 18" of boom at 5760. For sure the scaled elements from 
1296 would be fiddly to make and easily crushed at 3456 and 5760 though 
there is a Basic program about (printed in an earlier microwave handbook 
thee three volume version) for computing different element parameters 
and bands.

Or to match the dish gain as listed by Donn, the 2.3 GHz looper would 
only need to be about 18" long, maybe 4' long at 3456`, and it might 
take a couple each 3' long to approach the dish's gain at 5760.

Its a trade off situation, where having individual band antennas can 
speed band changing, but makes on site erection slower. Where the gang 
of loopers make give better performance at the cost of greater antenna 
space volume and more difficult aiming. It might be worth looking at a 
1m offset dish to capture more gain at the low bands if it can be 
carried, erected and controlled. It probably would be beneficial at 10 G 
to sacrifice some gain and underfeed the 1 meter dish to make for easier 
aiming. I think there's been something written recently about feeding a 
dish with the Vivaldi, I'll look for it after while, likely a recent 
magazine or MUD proceedings. I have the feeling that the Vivaldi as 
supplied by Kent is more appropriate for a short focal length prime 
focus dish than the Direct TV style offset dish. Because the pattern of 
the Vivaldi is more like a dipole than a multiple wavelength horn.

But then any antenna is better than staying home if you can make contacts.

73, Jerry, K0CQ

On 3/15/2011 5:22 PM, James French wrote:
>
>
> On Monday 14 March 2011 22:42:03 Dr. Gerald N. Johnson wrote:
>> There have been dual band horn feeds made for the Direct TV dish that
>> work well, most often 5760 and 10 or 10 and 24 because at 3456 and 2304,
>> the 18" dish doesn't outperform a loop yagi by enough to make it worth
>> the effort to try.
>
> But wouldn't the dish be better to use if you are getting the same gain as a
> loop yagi that takes up space and can get bent /destroyed easily?
>
> How many elements are you using to for the 2.4/3.4 beam to make the
> comparison?
>
> If I can save some space in the car and not have to realign loops, I would go
> with the dish personally then. I currently have no way to mount beams to the
> car until I stop and set things up. No racks to use on top for mounting.
>
> James W8ISS
> ______________________________________________________________
>