Sunday, 24 March 2019

New cable

I spent some time pulling-in a new coaxial cable to use on transmit between my PA and the QO100 tx antenna. The existing cable seems to be a bit unpredictable. The new LMR600  cable still needs to be terminated. A job for tomorrow.....
I know fairly reliably the gain of the antenna. I can measure the loss of the LMR600 at 2.4GHz and I  can measure my transmit power.
By transmitting a carrier to equal the lower beacon level, I  can compare my system against the published parameters for the narrowband transponder. As the transponder parameters were changed this last weekend It will be interesting to see how far the transponder parameters now varies from those original figures.
The nice thing about the LMR600 is that it is rigid enough to be forced through the now-congested duct into my shack!
My 738 to 144MHz downconverter is now GPS locked so I can see, with the upconverter and LNB also being locked, that the FT847 has about a 350Hz offset between 144 and 432MHz. That is getting a bit much and needs to be corrected.

QO100 transmit (uplink) antenna


Friday, 8 March 2019

LNBs for QO-100

Although I have an Octagon with 25MHz reference, injected externally, my second Octagon has a 27MHz reference. It seems to be difficult to obtain a consistent supply of the same type of Octagon
I was keen to try another brand of LNB and having seen that the Goobay 67269 sells on Amazon for £5.43 and uses the NXP chip (guaranteed 25MHz ) TFF1017, I ordered one. On opening it I confirmed it has a 25MHz crystal reference. I haven’t tried it on air yet, but I plan to try it on my smaller, portable ‘Sky’ dish. If successful I will modify it for an external 25MHz (or other frequency) reference input, or maybe one of the SMD TCXOs I bought a while ago!.
The NXP LNB chip should be amenable to a range of reference input frequencies (within reason) opening up the possibility of a 2m or 70cm IF.
Having had success with one Goobay LNB I ordered two more to check out whether it was a repeatable source. The two arrived and were duly opened and checked as both 25MHz reference. They were.
Unfortunately, the Goobay 67269 only has one F socket port, so the possibility of having simultaneous horizontal and vertical polarisation is not possible. Nor can a second port be used for an external reference input. However, the large cavity under the  board, currently used to accommodate the 25MHz crystal, is ideal for mounting a second socket for this purpose.


Tuesday, 5 March 2019

QO-100 developments

I have been using a number of different antennas for the uplink on 2.4GHz.
As I am unable to achieve much more that 1.2W output from my SG Labs transverter, my tests have nearly all been done with 1W or less.

1W into the cable out to the 17.5dBi gain Huber Suhner flat plate antenna results in a signal that most QSO partners report as a just a bit too weak for comfortable copy (<10dB SNR in 2.5kHz?). Using my 2.3m EME dish with 13cm band septum polariser feed requires that I turn the power down to around 100 -150mW to reach beacon level. The septum polariser feed is probably not working so well at 2.4GHz. It was designed for 2320MHz. QSOs are easy with this set up, though. No PA required.

I have built a 4 turn helix to the W1GHZ online antenna book details. The match is as shown below. Excellent!

This shows almost 40dB return loss at 2.4GHz, although in practice keeping the spacing of the matching strip to the reflector is difficult and when set up for more testing, 20dB was about the practical limit. Once Dow Corning sealant was added to hold the helix in place, the RL had dropped to about 17dB. I judged this adequate for testing and with the helix temporarily taped above the LNB in the 1.1m offset dish, it was hard to get much a over 10dB below the beacon level, even at 1W into the feeder. The estimated (not measured)  cable loss to the feed was about 4dB. It was obvious from results that a bit better optimising was required.
With this set up I called and worked 3B8FA on the satellite. He was clearly having trouble copying me, so I completed the QSO using the EME dish!

Due to the kindness of one of my local amateurs I will be taking delivery of  a set of metalwork for one of the G0MJW dual band patch, dielectric lens feeds to try in the (near?) future.

One of the joys of this satellite is that I have already had QSOs with friends I haven’t worked on the radio for some time. Everyone seems to be having a go at working the satellite.
I worked Per, DC3ZB earlier today. The last time I saw Per was at Friedrichshafen a few years ago. Lots of time to chat and compare systems and news.