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

[Dr. Gerald N. Johnson]

Kent's Vivaldi is pretty elementary, single sided PC board, several 
sizes. The larger ones are shown at: 
http://www.wa5vjb.com/images/3400MHz-hi.jpg and his feed line 
connections on the higher frequency model at: 
http://www.wa5vjb.com/pcb-pdfs/10-25GHzSweep.pdf Kent terminates the 
lines with a circular opening in the metalization behind the coax 
attachment point.

Multiband feeds for any dish are a compromise because any practical feed 
for a wide range of frequencies moves the phase center (the point where 
the center of the radiated spherical wave is) with frequency. You set 
the focus on the highest band because its fraction of a wavelength 
sensitive and you lose less on the longer wavelength bands.

Part of the compromise then is the beamwidth of the feed can also vary 
with frequency and each reflector needs that optimized. The F/D of an 
offset dish makes it a lot easier to feed than the typical short focal 
distance of the common prime focus dishes. The offset dish F/D runs 
about .7 while popular prime focus dishes run more like .35 to .4 
needing a feed with 120 degree beamwidth and sharp cutoff. The feed 
beamwidth for a .7 F/D dish is more like 70 or 80 degrees and its a lot 
easier to get rapid cutoff at the dish edges.

When you cover 3.4 to 10 GHz, with a manageable dish at 10 GHz about 18" 
(gives about 5 degrees beamwidth which strains manual aiming agility in 
both azimuth and elevation), you get about 32 dBi gain. With gain 
proportional to dish area in square wavelengths, 1/3 the frequency, 3 
times the wavelength, 1/9th the reflector area in wavelengths, you get 
about 22 dBi at 3456, comparable to a workable looper.

When roving you rarely have a level surface to park on so you have to 
move the antenna in elevation and azimuth and going bigger to make up 
for an inefficient multiband feed makes aiming on the higher bands a lot 
more difficult. Unless that multiband feed underfeeds the larger dish on 
the higher bands to widen the antenna pattern which means a more 
directive feed on the higher bands. Most attempts at a multiband feed 
desire to maximize gain on all usable bands. So a stepped circular 
waveguide feed with a constant aperture might be more appropriated with 
the feed beamwidth narrowing as you go up in frequency. Possibly a 
relatively long tapered horn with several feed points of probes about a 
quarter wave above where the taper cuts off as a waveguide. Or to clean 
up radiation, a couple ridges in a feed horn in the Vivaldi shape. There 
are commercial wide band horns made that way. But while their impedance 
match is pretty much independent of frequency I think their directivity 
will increase with frequency, which might be beneficial in the multiband 
feed for the rover with the larger dish underfeeding it on the higher 
bands to keep antenna directivity manageable by hand.

I was scanning through the 2011 ARRL Handbook yesterday afternoon and 
there is discussion of multiband feeds for dishes in there and a picture 
of one of the PC board feeds smoked by too much RF power.

Where there is more microwave activity than here near the Boondocks, it 
probably isn't necessary to squeeze the last 10 dB out of the rover 
antenna to make contacts either.

73, Jerry, K0CQ

On 2/3/2011 10:08 PM, Jon Ogden wrote:
> I spent an entire summer in college (20+ years ago)  one year working in an anechoic chamber doing pattern testing on a Vivaldi antenna.  The thing had bandwidth out the ying-yang.  Was absolutely beautiful.  However, the entire feed structure radiated like a sieve.  To compensate we put absorber material around the feed to minimize the radiation off the feed so we could see what the antenna really looked like.  It helped some but not that much.  I imagine that simply due to the physics of a  wideband PCB horn (which is really what it is) that you'll get some sort of radiation off the open transmission line structure.  Maybe WA5VJB ended up doing the antenna and feed in stripline (ours was just single layer microstrip).  Doing it in stripline would certainly help to contain the feeds on the feed structure.  Makes feeding a little more difficult but it would help stop the radiation off the sides of the antenna.
>
> But let's put it this way:  This is a truly wideband type of antenna design.  It's definitely a wider band design and an Log-Yag.  It's a slick antenna and would probably work well for you provided the feed problem I saw was fixed and it probably has been since that was a long time ago (and hey I believe it was 1987 or 1988 - the Iran-Contra hearings were on then - I remember listening to the in between tests (radios didn't work too well inside the chamber!).
>
> Jon
> NA9D
>
> On Feb 3, 2011, at 6:02 PM, David Palm wrote:
>
>> I have been corresponding a bit with WA5VJB about the use of one of his
>> log-periodic yagis as a multi-band feed for a dish.  (He actually
>> recommends  using one of his newer Vivaldi PCB antennas, which he says
>> should be a bit easier to connect and a bit more efficient in this role.)
>>
>> I realize that this is at best a compromise solution on every band, but it
>> is extremely attractive to me for a rover set-up to be able to use the same
>> dish from, say, 2.4 - 10 GHz.  I was thinking that I'd try to build a loop
>> yagi for 2.4 GHz and try using the multi-band dish on 3.456 MHz and up (if I
>> get the equipment working in time.)  But you get the drift.
>>
>> I've read the article that Paul Wade, W1GHZ wrote about this (
>> www.w1ghz.org/antbook/conf/WA5VJB_LPA_feed.pdf) and the two take-aways from
>> that were that 1) it should work and 2) it seems best to optimize the system
>> for the highest band and then just take your lumps on the rest.
>>
>> I'm wondering if anybody has any experience on this that they could share?
>> I'm interested in having something ready for the August contest and am
>> starting to plan now.  If you have other suggestions for multi-band antennas
>> for the rover, I'd be very interested to hear your suggestions.
>>
>> 73,
>>
>> David  W9HQ
>>