Mounting of POTY on dish, SWR high, how to fix?

    Quote from DD4YR

    Yes, :thumbup:

    to lengthen is the better idea, and you have more place/distance with the POTY reflector disk to the feed arm.

    You have also a wider area to slide the POTY in direction to the dish reflector to find the best position for focus.

    Here is what I came up with last night. With this setup, I could hear the beacon and some QSOs, but signals were weak. On TX, I could not see my signal on the waterfall. So more work needed.


    My guess is that the problem is the position of the feed. The feed is now the same distance from the feed as it was in the original, where it was working well, but in the original the bottom of the reflector was touching the mount, now it is about 2 cm above, so it is not pointing to the same spot on the dish. Any way to fix this?


    73, Bill

    Hello Bill,

    the distance from the POTY to the dish reflector is not critical for 2,4 GHz. You have to adjust with the 10 GHz RX signal.

    Your mechanical solution looks good, but in vertikal to satellite has changed.

    So you have to adjust the elevation from your dish.


    take a look here:

    http://www.w1ghz.org/antbook/chap4.pdf

    Figure 4-9 Loss due to axis feed displacement from focus.

    Oh dear - there is usually a much simpler way to make it fit on a standard holder Just replace the top plastic part with a much thinner bit of metal. Done. Not this but hopefully you get the idea.


    [Blocked Image: https://images-na.ssl-images-amazon.com/images/I/51bisN884sL._SL1200_.jpg]

    But really think it is plumbing so use readily available fittings. One of these mounted on the boom.




    [Blocked Image: https://plumbingsuppliesdirect.co.uk/wp-content/uploads/2018/03/Stand-Off-Pipe-Clips-Multiple-sizes-Finishes-1.jpg]

    Mike

    Quote from OM3BD

    Here is what I came up with last night. With this setup, I could hear the beacon and some QSOs, but signals were weak. On TX, I could not see my signal on the waterfall. So more work needed.


    My guess is that the problem is the position of the feed. The feed is now the same distance from the feed as it was in the original, where it was working well, but in the original the bottom of the reflector was touching the mount, now it is about 2 cm above, so it is not pointing to the same spot on the dish. Any way to fix this?


    73, bill

    The simple way to fix the height change to grab one side of the mount firmly with one hand, the end of the boom with the other and bend the boom to the right spot. Most LNB supports for inexpensive dishes are suitably flexible to do this.


    Mike

    Quote from G0MJW

    The simple way to fix the height change to grab one side of the mount firmly with one hand, the end of the boom with the other and bend the boom to the right spot. Most LNB supports for inexpensive dishes are suitably flexible to do this.


    Mike

    Thanks, Mike. It has been raining non-stop for 2 days, if it would just stop raining for 30 minutes, I could take the radio outside and watch the meter for best reception as I adjust the dish.


    @G0MJW

    @DD4YR

    @DH2VA

    POTY high SWR?


    It finally stopped raining so I brought the radio outside and repaired the dish, which was not damaged. By fine tuning the azimuth and elevation, I was able to get better signals on rx, -35 on the beacon on the gqrx dBFS meter with just the LNB, -38 with the LNB mounted on the POTY (no lens). Is that about normal? I'm familiar with the dBm S meter scale on the web SDR but I've never before seen a meter with a "dBFS" scale.


    Anyhow, I was happy until I tried to transmit. I did not see my signal on the downlink. I opened the amp and I'm reading 0.34v on DRV, 3.3v FWD, 2.3v on REF. So it seems the SWR is too high. There was only 0.1v on REF with the BamaTech feed. So it seems there is an issue with the POTY. Is it possible to adjust it for lower SWR? And what are the mechanical tolerances? I see on the instructions that there is supposed to be 3mm between the reflector and the DE, on mine it is closer to 4mm, is that a problem. I'm also not sure the two pieces of metal are 100% parallel to each other.


    Thanks for any advice and 73,

    Bill

    Hi Bill


    The POTY spacing and dimensions are critical for it to work. 4mm spacing is a 25% error so far to far out. I would suggest re-soldering it with a temporary 3mm spacer to get it correct. Then you might need to bend a little to get best match. Starting from 4mm spacing would not be good. Perhaps you can show a more detailed image ? The critical spacing is the gap between the plates but also the way the connector is mounted matters. There should be no screws coming into the gap.


    Mike

    • Official Post

    dbFS is decibel relative to the full scale of the SDR receiver (which is max sum amplitudes of all signals to the SDR input). It is rather pointless to give any numbers here as these will vary depending on LNB, cable used, temperature to only name a few.


    The ONLY way to properly max the antenna pointing is to max on any of the beacon signal levels, regardless of the reading itself. Then to compare different setups to each other you have to check the transponder excess noise referenced to the thermal noise outside of the transponder BW. Select a display BW of at least 3 MHz and you should see a hump about 600 kHz wide and 3-4 dB sticking out of the noise floor left and right. This is the transponder passband and the 'stick out' value is the only one which is independent of the SDR software used. For 80cm, 3-4dB is fine, if you don't see this I would not touch the TX and figure out RX first.

    Quote from DH2VA

    Select a display BW of at least 3 MHz and you should see a hump about 600 kHz wide and 3-4 dB sticking out of the noise floor left and right. This is the transponder passband and the 'stick out' value is the only one which is independent of the SDR software used.

    Yes, I am seeing the hump and signals are strong. Looks like the rx side of my setup is good.

    Quote from G0MJW

    Hi Bill


    The POTY spacing and dimensions are critical for it to work. 4mm spacing is a 25% error so far to far out. I would suggest re-soldering it with a temporary 3mm spacer to get it correct. Then you might need to bend a little to get best match. Starting from 4mm spacing would not be good. Perhaps you can show a more detailed image ? The critical spacing is the gap between the plates but also the way the connector is mounted matters. There should be no screws coming into the gap.


    Mike

    Mike,

    Here are a few more photos. The prior owner says this POTY has been tuned and tested. I measured just under 4mm with a ruler, I don't have calipers, the measurement could be off. Also, the only available coax I have to connect the PA output to the POTY is a 10m length of H155, that probably loses half my output.


    I just tried the BamaTech feed again and it is still working, I see my downlink and swr is low.

    It looks well made. A ruler will not be accurate enough. Do you have a 3mm drill bit for example?


    What you need to do is find something 3mm thick (exactly, not approximately) and put it between the plates to measure the spacing. A metal shim is good for this. If it is not 3mm then you need to make it 3mm by resoldering the patch with a blowtorch or hot air gun. If you use a damp cloth behind you can prevent the reflector and connector from unsoldering while you melt the solder on the patch.


    Some people found a slightly under 3mm (say 2.9mm) aluminium shim between the plates is a good way to get a 3mm spacing. The difference allows for it not being easy to ensure it is tightly compressed together when at 350c. When that's done you should have a good match but if not it is possible to adjust be very slightly bending the patch. However you really need to have access test equipment to do this. They are only $60 for a Nanovna working to 3 GHz, but I recognise that unlike in the UK, you can buy a lot of beer for $60 in OM, so perhaps the local radio club has something they can lend?

    Quote from G0MJW

    It looks well made. A ruler will not be accurate enough. Do you have a 3mm drill bit for example?

    It looks like the problem is coax or connector, not the POTY.


    The POTY has an N connector, the Bamatech has an SMA. I have a 10m length of H155 with an N connector and a 0.5 length with an SMA. So I used the 0.5m with the Bamatech and it worked, strong signal on downlink, swr good. I used the 10m with the POTY and no success, could not hear my signal on downlink, high swr.


    Today I got an N to SMA adaptor and for the first time was able to try the 10m coax with the Bamatech feed. And the result was the same as with the POTY - nothing on downlink, high swr. This was quite a surprise because the 10m coax appears to be high quality Belden with professionally installed connectors. I'll have to find another coax and hope that works. I thought perhaps 10m was too long and lossy but my friend also has 20w PA and 10m of H155, and his signal is so loud he has to reduce power to avoid Laila.


    Anyhow, I'll try the POTY on a fresh coax and hopefully have success without having to resolder or bend.


    In the meantime, I am FINALLY qrv on QO-100 using the Bamatech feed and the 80cm dish for TX, and a second dish with just the LNB for RX. But since for now I just have it working with the 0.5m coax, I must bring the radio outside and sit next to the dish to operate. I had several nice QSOs today with good reports. But it will be better when I can sit inside in the nice warm shack.


    73, Bill