During the very enjoyable CAT25 event last weekend, the subject of dish antennas suitable for HamTV came up and the newi(ish) 700mm three petal Discovery dish was mentioned.
As I had one sitting unused in the garage, I agreed to do some investigations.
The item is intended for receive only use one variety of frequencies between 1 and 2.5 GHz. and is now readily available from Mouser https://www.mouser.co.uk/c/?marcom=122652156
The support for a patch or helix could either be with a homebrew tripod arrangement or through the use of a 20mm UPVC water pipe. Thanks to how ever made that suggestion in the bar or over dinner!
As can be seen, the 20mm o/d pipe is a perfect fit to slide inside the POTY 22mm copper pipe with the lens removed. This shows the general arrangement
https://postimg.cc/xXKkHzpk
One question related to the size of the perforations. I measured these at 5mm , this compares with a Sky dish of around 1.5mm. 5 mm is theoretically too big for 10GHz operation but see results below. I believe that it would be quite easy to apply some adhesive aluminium foil to the ?rear face of the dish if required.
Comparisons were made between this dish and my adjacent 1.2mm offset dish. Theory suggests that difference between the two dishes should be around 5 or 6 dB.
Finding the focal point was simply found by sliding the pipe in and out!
On the QO100 uplink, using the same tx and identical feed lines, 20 watts was required with the small dish to provide the same signal at the transponder as a 6 watt transmission on the big dish...
On the QO100 downlink, the beacon signals were indeed approx 5 dB down - so it seems to work ok on the higher frequency.
I tried "hand holding" a POTY dish at the focus and, on receive, similar results were obtained.
So conclusions:
Easy to transport and lightweight. Seems to work well with a POTY
Probably ok for the middle part of high elevation HamTV passes - perhaps 5 or 6 minutes. More research to be undertaken when I get the dish up the mast!
Fine for portable narrow band QO100 portable operation.
Probably not enough gain for DATV QO100. operation.
The manufacturers have a small elevation rotator in the works https://www.crowdsupply.com/krakenrf/kr ... e-campaign
NB: My measurements were intended to be indicative not lab standard!
CAT 25 Dish discussions
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Re: CAT 25 Dish discussions
That would have been me Graham.... I played a quick video of it doing SSB
While quite significantly down on my 2.4m dish, SSB was fine. I doubt it will be large enough for DATV. The 2.4 GHz gain figures can be relied on but on receive the SNR is a combination of the uplink and downlink SNR. In practice this means a 2.4m dish is maybe 1dB better than a 1.2m dish. This is because the transponder noise floor is dominating the link budget with the 2.4m and while less so, still pretty much with the 1.2m. With the 68cm the transponder noise floor will be less of a factor, bit I would expect less than 6dB difference. Going from 68cm to 30cm I would expect to lose 6dB. We know Graham's little 30ch dish works well, I would compare it against that.
Mike
- PetalDish.jpg (273.58 KiB) Viewed 74 times
Mike