Using the SDR-IQ and continuum mode to measure Sun/cold sky ratio

(Or how to gain 2dB of Sun noise without even trying)

Since I started EME I have always wondered why I never got close to the sun/cold sky results with my system compared to VK3UM's predictions with EME calc?

I use an SDR-IQ and Spectravue in continuum mode on the 8.25MHz 190kHz wide IF of my K3.

 

 

In the last hour or so I have been playing with the SDR-IQ IF and RF gain settings to try and minimise some spikes that are picked up at IF from my DRIACS controller when it's doing PWM.

I normally have the IF gain set at +24 and the RF gain of the SDR-IQ at +10. i.e flat out!

Have you guessed what's coming next?

Sun/cold sky at 6cm from VK3UM  =  10dB

Sun/cold sky at 6cm with original high gain settings = 6.5dB

Drop the RF gain to -10dB and IF gain to 0dB and

Sun/cold sky at 6cm with new settings = 8.5dB!

Clearly overloading the ADC in the SDR-IQ all these years! D'oh!!

It also might explain why I never really ever got a sharp peak in Sun/sky when I adjust my feed positions. I'll confirm that tomorrow!

You live and learn!

24GHz anomalous propagation

Since 1114 UTC today, and despite no visible trace whatsoever on the WSJT-X waterfall display I've been getting continuous, averaged decodes from GB3PKT's 24GHz JT4G signal via "Drizzle scatter" :-)

This is over a 77km, totally obstructed path from Sea level here on the Fen Edge to Sea level at Clacton over the "Essex/Suffolk Alps". Signals vary from -19 to -21dB.

So much for the "experts" who that say that 24GHz is a water absorption band and only works when the atmosphere is dry.

I'm now monitoring the beacon 24/7 using a Raspberry pi running WSJT-X and a Funcube Dongle behind my transverter and it's popping up all sorts of interesting propagation. Followers of my Twitter feed @g4bao will have read my reports of "late night, after a hot day" propagation last week.

We need more home stations on the band (or portables prepared to get wet!) to experiment with these modes and make some digimode QSOs on this seriously under-used and underestimated band.

In a recent "GHz bands" I reported that G8ACE and G4LDR have shown that this type of drizzle Scatter/ Cloud scatter propagation (call it what you will) even works on 47GHz with decent eqiupment.         

You miss SO much by only coming on during contests and activity periods and working the "usual suspects" over the usual line of site paths. Do the sums as to how the probability of anomalous conditions falls when you operate on the fixed days determined by the contest committees! Anyway in these days of "No Tropo" it's something to keep you interested!

Here ends my appeal for more experimentation than competition!

Christmas project - Can I injection lock a Mk2 DB6NT transverter?

The excellent G4DDK Anglian transverter range has the facility to "injection lock" it's 116MHz Butler Crystal oscillator to a stable 116MHz source such as a synthesiser. This allows the whole transverter to be locked to a stable reference such as a 10MHz GPSDO. Injection locking does this but still retains the clean, low phase noise of it's Butler Crystal oscillator. Using just the synthesiser as an external LO would have meant that the whole transverter's noise performance would have been governed by the synthesiser's phase noise performance. Using this method, the crystal is just "nudged" on to the exact frequency by the synthesiser.

This got me wondering if I could do the same with the single FET oscillator used in all the previous-generation Kuhne G2 series of microwave transverters. I have tested this out on a 5.7GHz G2 with a 117MHz crystal LO, but the oscillator circuit used in the G2 is common to all the Kuhne transverters below 24GHz so should work in all of them given the correct crystal frequency.

Most microwavers know that the G2 series have the facility to remove the crystal and inject an external LO. It's a standard mod used by Kuhne themselves in the "external LO" version of their oscillator chains. The injection point is via a 100pF to one end of the crystal position, so I tried to inject a signal at that point while the crystal was in position and oscillating and see what happened.
I used  a +5dBm from a G4JNT LMX2541 fractional N synthesiser. This board is simple has good progamming support and has an on chip VCO. It is not though, in the top class for phase noise!
I monitored the transverter LO at the output of the first  tripler at 351MHz with a spectrum analyser and a Rubidium - locked frequency counter. As you would expect, a nice clean signal for the crystal but slightly LF of the required frequency.  On connection of the external 117MHz synthesiser,the oscillator immediately locked up showing 351.0000MHz but sadly the output noise spectrum immediately degraded to match the synthesiser, not the nice clean crystal. I reduced the synthesiser level and watched the spectrum and frequency. As I reduced the synthesiser drive level, the crystal stayed in in lock and the output noise reduced.
"result!"
With about 20dB attenuation - a drive level of -15dBm, the frequency was still 351.0000 and locked but the noise now looked like the unlocked crystal.
So there we have it.
Without removing the crystal, just like the Anglian, you can injection lock your Kuhne G2 transverter to a synthesiser such as the LMX2451 or ADF4350/1 and not spoil the phase noise performance!


Have a Happy and quiet (low phase noise)  Christmas!

73 John

Isn't being an Engineer brilliant?

This afternoon I've been working on the switching for my 5760MHz EME dish feed electronics. Nothing complicated, just a couple of high side MOSFET switches. A 300ms delayed 12 V high current one and a switched, non-delated 28V rail to activate the coax relay. Standard P channel switch circuitry with the 12V  input to the Source of the FET and the output from the Drain. 6.2k resistor from source to gate to keep it off and a NPN transistor from Gate to Ground to turn it on with. CR delay circuit in the base of the driver from the 12V TX out from my DB6NT transverer.  Worked fine, so I used the same circuit for the 28V one but with no delay ahead of it.
Built it, connected it up tried it and BANG! the driver transistor fried. Transistor was taking over an amp and current limiting the 28V supply .FET had also gone phut. Gate Source short circuit dumping the full 28V across the driver transistor. 
Whoah! Why do that with the 28V supply and not the huge 8A 12V supply?
Much head scratching and a couple of changed devices later (good job I have apenty of both)  I thought I'd better check the datasheet of the FET. Yep, maximum drain to source voltage was 60V so that's OK. Ah..... maximum drain to GATE voltage +/-20V Lower than VDSmax.... I didn't know that!
Grounding the Gate in this circuit with a transistor would put 28 - Vcesat = 27.8V   from Source to gate. Yep that would be more than 20V then. Failure would short gate to source connecting the full 28V across the collector to emitter juunction of the driver transistor taking that out as well!

I  do like to understand why things happpen.

Solution? Add an 8.2k between the transistor collector and the FET Gate. Limiting the Vgs of the FET to 28x8.2/14.4 = 15.94 V below 20V now so that's fine.

Reason for the title of the Blog? Every problem is a learning experience, and Engineers are all about solving problems.     Ergo, Engineering is all about learning.... BRILLIANT!! !
 
   

One volunteer is better than ten pressed men!

With the Martlesham GHz Bands Round table and UKuG AGM coming up soon , the organisers, the Martlesham RS were disappointed to learn that top EMEer and the French half of the UK 10 GHz record QSO, Guy, F2CT has had to pull out of attendance and his talk due to work commitments.

Guy, F2CT/P with his record breaking portable system

 I was discussing this with talks organiser Sam, G4DDK and within a few minutes I found myself as Guy's stand-in!
I plan to give a talk entitled "Using your 2300MHz NoV - options and ideas." I'll be cheer leading for use of our new 2300Mhz NoV and presenting a number of options for getting going on the band and re-using existing 2320MHz equipment.  I'll cover the modifications to an Anglian 144Mhz transverter and some ways that popular commercial equipment and homebrew can be modified for the band.

Hope to meet plenty of you there!     

The Bodger's Guide to multiband antenna stacking

Recently a local who's building a 1296MHz station asked me what seemed at first glance like a simple question, namely, "how far apart should I space my 1296MHz antenna from my 144MHz antenna ?
My reply was there are two answers, the "correct and academic" one and the "I don't have a 25m tower" answer. The former is covered in great detail on g3sek's excellent ifwtech.co.uk  website and other places I'll mention later and the latter is "as far away as is practical to put it given your space!"

Now unlike many active stations on VHF and up, here on the Fen Edge I live in a modest house, surrounded by neighbours, so a tower is a non starter and all my Terrestrial antennas live on two poles, one on each gable end. I manage 2 VHF bands, and at least 3 microwave bands with this system, but compromises have to be made. A single 60cm dish does two GHz bands without compromise (except maybe height agl) but the boom of my 1296MHz Yagi is only about 500mm above the boom of my 50 and 70 MHz YU7EF dual bander. They both "work," the VSWRs are not compromised and I get excellent results on 1296 and can have Sporadic E fun in the Summer on the lower bands. That's really all this hobby is about to a certain extent.

 My Good friend Kent Britain WA5VJB (Google his name and you'll see he's a "proper" antenna engineer as well as being a bit of an iconoclast) authored an excellent paper in the 2010 Central States VHF conference proceedings about close stacking and is a "must read" on the topic. In it he states that UHF antennas can be stacked "closer than you think." Unfortunately that paper does not seem to be on-line anywhere obvious, but it was covered in Winter 2010 "CQ VHF." Gerald Johnson K0CQ's follow up paper takes the usual "Dr Jerry academic approach" and is worth a read. 

I've always put my antennas as far apart as I can here, and with me that's very close indeed as I only have a short pole on the gable end. I have serious limitations  when winching the pole up past the house eaves. 

Honestly, I wouldn't die in a ditch about it, and take the pragmatic view for terrestrial that as long as the VSWR is not seriously compromised by proximity just do it! 

Or don't sweat about what you can't control! Live with it! 

Report on the Dubus 23cm CW contest weekend

This was my first experience of a 23cm Dubus CW event and I had great fun! I'm not sure why there is so much concern about the demise of CW, it seems alive and well on 23cms! If only we could encourage more activity outside the contests. Only surprise was the lack of Americans, I didn't hear one the whole weekend! 

Running just 120 Watts (my power seems a little down) to a 1.9m RF-Ham design dish with the excellent SM6FHZ patch feed and a 0.2dB G4DDK VLNA23 I worked 19 stations on CW and 2 on JT.

23cm Initials were HB9CW, SP7DCS, OK1CA, SP6JLW, OK1KIR, ES5PC, OZ4MM, F5SE/P on CW plus G4DDK and ZS6JON on JT.

Notable "got aways" were PA3DZL, DL1YMK, DJ8FR, and DL6SH Jac was quite weak, but the latter 3 of that list were stronger than many I worked but just didn't hear me, despite endless calling. Most unusual!

I found that "search and pounce" worked much better for me, calling CQ brought some QSOs and a couple of stations that were just too weak for me to work, which made me question the reciprocity of the 3 in the list above, or maybe it was just QRM?

The slow QSB was very different to what I've experienced on 13cm with the fade rate being "just right" to take out Morse characters!

In all, great fun, and it made me think that the "demise of CW" that some bang on about is just down to the problem even the terrestrial microwave bands suffer from, namely  lack of people bothering to come on and transmit, instead, checking the internet, seeing no activity and not switching the rigs on!