Posts by G3WDG

    Experiments with helix element diameter

    I did some sun noise tests recently on a 0.33 f/D prime focus 1.2m dish at 10GHz, using two different diameter wires for the helix. The 10GHz element of the feed is a 22mm diameter water pipe squeezed in a vice to fit inside a WG16 (WR90) flange (to connect to my DU3BC preamp).

    The feed is shown below:

    Sun noise measurements:

    helix element not fitted = 8.5dB

    1mm diameter helix wire = 8.0dB

    2.75mm diameter helix wire = 7.0dB

    These measurements seem to confirm that the reduction in performance on receive is caused by blockage from the helix, and that thinner wire is better.

    The 1mm wire helix was no problem to match to 50ohms using the quarter turn strip per the G3RUH design, and seemed to have a good axial ratio.




    Nice description!

    I agree it is generally not possible to reverse calculate the thermal resistance from chip to outside casing of an amplifier. It is hard to do accurately even as the designer of the amplifier. I would guess that for a professionally built amplifier, unless you have the manufacturer's spec to hand, then 60C maximum baseplate temperature is a good figure to aim at.

    I have also used water cooling of SSPAs with great success. It is not too difficult - a copper block with copper pipe soldered to it and a bucket of water and fish tank pump works pretty well, but I have not measured the thermal resistance of this.



    Just for fun, trying a small antenna today in JN86PQ. Its is a 20dB horn, indoors behind a double glazed window. Can copy a few ssb stations. CW beacon is easy copy and have managed 5/74 good CRC blocks off the PSK beacon. FEC blocks are solid copy.


    Charlie HA/G3WDG

    While this is true for small signal amplifiers, power amplifiers don't quite behave the same as far as large signal performance is concerned. There will generally be an optimum load impedance (ie output tuning) that sets the optimum power and this is not affected (at least at the fundamental frequency) by variations in input tuning.

    The procedure that I find works pretty well is to start with the output tuning as is, then tune up the input with a small signal (eg 10dB below what you would need for max output) for maximum gain. Having done that, move on to the output tuning with more drive, say about 3dB below that to give saturated output. Aim for max power or max efficiency as you want (max efficiency is often not far away from the maximum small signal gain tuning point). Once happy with the output tuning, increase drive further and repeat (small improvements now only likely).

    Once the output tuning is optimised, move back to the input with drive backed off about 3dB from max saturation. Small improvement in gain may be possible.


    Charlie G3WDG

    Hi Achim

    As expected, WB beacon dropped out here on my 76cm dish, which gives an MER of 6dB under clear conditions. Sun noise was measured yesterday as 5.8dB with this dish.

    At 1012z MER started to fluctuate between 6 and 5, with sun at 146.4/24.3 deg az/el. Satellite is 147.7/25.3.

    MER continued to degrade and I lost video at 1016:30 with sun at 147.5/24.7

    Video back at 1024:31 with sun at 149.6/25.3.

    MER back to 6dB at 1030 with sun at 151/25.7.



    Hi Matteo

    Three different software packages.

    Spectravue for sun noise measurements, with homemade SDR based on Softrock. I'm not sure if this will work with RTL based SDR's. Hopefully someone else on the forum can advise.

    Minitournier for measuring MER of the WB beacon.

    RSP-SAS Spectrum Analyser in conjunction with RSP1A SDR for comparing the WB spectrum above noise.



    I just confirmed the measurements using Sun noise. With helix fitted sun noise was 4.5dB, without helix present it was 5.8dB. Correcting for S+N/N, this equates according to my sums to a drop of 1.9dB in system receive performance caused by the helix.



    One issue that I have been thinking about with helix feeds is blockage from the helix structure in front of the feed pipe.

    I have a simple feed here with 22mm water pipe protruding a few mm from the reflector of a 2.25turn G3RUH feed. The feeds works great on my big dish, but have been testing it today on a 76cm prime focus dish with approx 0.4 f/D.

    Two tests so far, measuring the spectrum and MER of the WB beacon. Both tests show significant degradation when the helix is present (I unsoldered it from the feed while the feed was mounted in the dish, changing nothing else). The helix's reflector plate was left in place.

    For marginal systems, particularly on the WB transponder, this would suggest that while simple to construct, a helix feed may be inferior to a patch. For the NB transponder I doubt if there is any discernible difference (from blockage) as the transponder noise floor is the limiting factor for most of us.

    Upper screenshots are with helix present, lower two are with it removed. Spectrum is at 5dB/div.

    There wasn't any measurable difference in beacon MER with the helix terminated with 50 ohms, or left open circuit.

    There could be advantage in moving the open end of the feed further into the helix to reduce blockage, but as I understand it that would move the phase centres of the 2.4 and 10GHz feeds apart, leading to loss in gain for one of them. So some tradeoff may be possible.

    Lots to experiment with these days!



    For the NB transponder, the noise floor is so high that I would not expect to see much difference in S/N of downlink signals. The situation with the WB transponder may be more noticeable as the transponder noise floor is at a lower level, so the sun may well cause a noticeable degradation.

    I'm testing a 76cm dish (for /P operation) and see about 5.5dB sun noise with it, measured using Spectravue continuum mode. The cold sky reference was at the same elevation as the sun. Clear blue sky conditions.



    I agree it is probably best for now to have FT8 located at dial frequency .600. This will place all FT8 'channels' in the correct segment, and would occupy a few kHz above .600. .610 dial frequency could then perhaps be a calling frequency for other narrow band JT modes such as JT65 and QRA64, with activity spreading (upwards?) as necessary.

    Busy commissioning a 76cm dish with dual band feed. Hope to back on the air shortly.



    I think at around the 1.7 to 1.8 time there was a change to the way the frequency table was stored. If you end up with an empty frequency table, the fix is to reset the frequency table:

    This should be a one time operation.

    Edit: Forgot to mention that some of the modes (FT8 and MSK144) changed to 77 bit messages in v 2.0, meaning that earlier versions of WSJT-X are now incompatible with the latest version.

    Also, the JT65 decoder in v2 is considerably more tolerant of drift.