Monday, 17 November 2014

ISCAT-A

ISCAT-A beacon signals successfully decoded from G4JNT's personal beacon on 2300.350 this afternoon. Once Andy turned the beacon antennas roughly towards me decodes started quite regularly. As London Heathrow is pretty much on this path I assume it was principally aircraft into and out of the airport that were responsible.))))))
I am very impressed by ISCAT-A for aircraft reflection compared to JT65C at 2300MHz.
Generally, if you hear the reflection burst, you can expect to have a decode. When ISCAT-A reports below about -20dB decodes are less reliable, although more testing is required. In a large number of decoded frames the number of erroneous decodes was seen to be less than a dozen or so over a three hour period.

I am now turning my attention to enabling the PA switching to work with the preamplifier box so I can try a two-way QSO on the new band. You need to be very careful with sequencing when switching 250W at 2.3GHz on a single coaxial cable with connection to a very low noise preamp!

Sam




Sunday, 16 November 2014

2300MHz beacon

IIt was good to see signals from the G4JNT 2300.350MHz ( new band) personal beacon this afternoon. The computer managed to decode several two and three minute periods. Reception was principally by aircraft reflection ( know as AS or AR) and easily identified by the fast Doppler. JT65C is not an ideal mode for AR ( I personally prefer AR rather than AS) and Andy has said he will change the modulation scheme to ISCAT-A later this week. This scatter mode is better suited to aircraft reflection ( Maybe!).

I would not have been able to participate in the tests today except that I had a weekend visit from G4FRE. Amongst the various radio jobs we got done, Dave re-flashed my Apollo 32 LO synthesiser ( Google either Apllo 32 or N5AC) so that it now thinks it is an Apollo 256 and now has 1078 rather than 1078.5MHz as one of the output options (1078 *2 = 2156 + 144 = 2300).
Dave also updated my K3 and P3 software to the latest version. I didn't know it had changed. Now I can use 10MHz as an IF for 472kHz.......

With Andy's personal beacon on 2300.350MHz it was possible to use my 67 element Wimo yagi to listen via my homebrew 13cm transverter and K3 transceiver. Currently the yagi is connected via 5dB of loss direct to the shack where a 0.3dB G4DDK VLNA13 feeds the transverter. I am tempted to NOT put the preamp at masthead as the engineering required gets complicated as two feeders plus power feeding needs to be provided. I am not convinced the effort required is worthwhile at this stage. But.......
Whilst Dave was here he loaded up my 40m inverted V on several bands, in turn, and connected his Anan10 in order to WSPR and see where we could get to. I don't have a record of where the signal got to but I was impressed with 10m and may well connect my own transmitter to the same dipole and have a go. It is months since I did any WSPRing on then on 40m.
I may also be in line to get a WSPR Ultimate 3 transmitter. Should be different!

Back to sorting the 13cm preamp interface. I can run around 250W in the shack. With 5dB feeder loss the coax is not going to freeze........

Sam




Wednesday, 12 November 2014

2300Mhz

I managed to get the Wimo 67 element yagi on test today. A judicious bending of the driven element away from the first director improved the match to over 30dB. This was measured on a HP network analyser with the connecting cable normalised out of the measurement so it is the actual yagi match.
A VLNA13 preamp ( 0.27dB) at the feed point allowed me to see sun noise and readily show the difference between cold sky and ground.
I terminated a length of LMR400 to use as the ( sole) feeder from shack to the yagi, up at over 12m.
Tomorrow I plan to connect the preamp, set up the 200W plus  HPA and then wait for someone else to turn up on the 2300Mhz band. That could well be G4FRE, now next week.......

Sam

Tuesday, 11 November 2014

Exercising my 146Mhz NoV

After buying a 'gaming' USB headset with boom microphone last week I set up my K3, with USA 144-148Mhz internal transverter, for use with FreeDV digital voice.

Set up was simple but getting the transmit level right proved a bit more of a problem than I expected.
At around 14:30 today, in tests, I managed to hear G4BAO on 146.900MHz, using one of the recommended digital channels at the top end of the new band. 

I immediately saw the characteristic 16 carriers of the FDMDV transmission from John. The SNR was not that high, but John's voice was unmistakeable, if a little Stephen Hawking like!
When it came to my turn to transmit I hit the soft PTT and spoke into the boom microphone. After adjusting the distance to the microphone so that the 'Frm MIc' waterfall showed the level to be about 1/2 of maximum, I sent a report and then I turned it back to John. He immediately confirmed successful reception. As the signal is nothing like we are normally used to it was difficult to give a useful signal report. This is one of the things we will have to sort out with digital voice! My initial stab is 33D where the D stands for digital distortion. Later, when I wound the mast up a few metres, the report was more like 45D

After switching to 1600 wide from standard 1600 we compared results again.  It would appear there was little difference except that the carriers occupied a bit more bandwidth. There was no noticeable improvement in quality. The signal was best described as distorted but intelligible at low SNR, improving to acceptable at higher SNR, corresponding to the peaks of QSB.

So, what are my impression of the FreeDV contact at the relatively low EIRP of 10dBW?
The multipath signal over the 78km path had a lot of errors. SNR was low and when we switched to SSB ( at 144.285MHz) it was obvious that conditions over this normally reliable path were not good. My SSB report to John was 53 to 55. There was lots of QSB.

I understand that a newer version of FreeDV, aimed at VHF, is under development. This will have a different voice model and maybe some Forward Error Correction (FEC). There is little need to restrict the bandwidth to 1.2KHz at VHF and maybe an increase in rate and bandwidth will be worthwhile when signals are distorted by VHF multipath and doppler spread?

I hope to work a few more stations on 146.900MHz and compare results.


Tuesday, 4 November 2014

Changing shacks

Now that outside temperatures are falling it is time to migrate back to the indoors shack. Although I have lots of space in the outside shack, it is very difficult ( and expensive) to keep warm when outside temperatures fall below about 12 degrees C
My solution two years ago was to establish an indoor shack in the spare room and run a couple of Heliax coax cables from a patching frame in the outside ( garage) shack, under the lawn and then along the fence line to the side of the house. As well as the two main coax lines I have a six wire screened control cable between the garage and indoor shack. I also have a third coax for HF that shares some of the common route but goes to the bottom of the garden. All cables enter the house through Huber Suhners lightning arresters.
With these few connections full remote operation is a bit of a challenge. I decided ( for now) against using e.g. Remote Rig or similar due to cost.
Last winter I ran my K3 as the 144MHz IF over one of the coax cables, with the various microwave transverters in the outside shack connected to the IF as required. The problem with this arrangement is that the losses in the long, concatenated, coaxial cables between the mast mounted transverters for several of the bands and the indoors IF were just too high and tended to radiate enough to be heard locally on 2m!
The solution this winter is to locate an Anglian 28/144MHz transverter in the outside shack and feed 28MHz IF over the coax. Yes, there is still a danger of egress and ingress at these frequencies, but the coax is pretty good at 10m.
Since the K3 and the Anglian 28MHz IF are separate ( transmit and receive are independent) and I didn't want to use up both of the coax cables between the two shacks, I intend to use a couple of 3dB splitter/combiners rather than relays in order to separate the transmit and receive paths. Tests with two MiniCircuits splitters and a 6dB attenuator have shown that the idea works well. 
The second coax cable will be used for a second system, with another rig.
PTT is activated over one of the screened cables and when the transverter is switched to transmit the Anglian +12V on transmit signal is connected to another control cable wire ( through a current limiting resistor) to light an LED in the indoor shack as confirmation that the transverter has switched.
I also use a Bird sensor on the 2m transmit output in the outdoor shack and feed the sensor output back over a third control cable wire to the meter ( still in the Bird housing) in the indoor shack in order to check that there is actually transmit output.