Obstacle in RF path close to dish - Effects? Alternatives?

  • Hello,

    after being motivated by my first RX attempts (thanks to lots of support in this forum), I started getting my TX path together. I received all ordered components last week, put them together in an on the fly setup on the balcony yesterday, and started my first TX attempts

    With a couple of successful CW QSOs, I managed to "limp" successfully over the NB transponder. Responding to a CQ call in SSB was unsuccessful, but at least the OM reported back that "the signal of the HB9 station" was barely readable. So I know at least that I am at the bottom end with my current setup.

    The 72cm dish is set up for best RX signal. When I mount the Helix feed on the LNB, the RX signal goes down by approx. 2 dB. I can hear my own TX signal on the WebSDR in Goonhilly weak but clear. With my own RX setup, I can hear that there is something in the noise. (QSOs are possible without cheating by using the WebSDR for RX). There is certainly room for improvement, but the RX path is sufficient for QSOs and has no priority for the moment.

    I use a SG-Lab transverter with 2W output. The short SMA to BNC jumper cable connected to the Feed with an BNC to N jumper probably attenuates an extra 2 dB (It's made in China, out of my "just in case" adapter box with adapters which I collected without any specific purpose) so with a proper low loss jumper and high quality connectors, I can probably recover 1-1.5 dB in the UL path.

    My biggest worry is the TX antenna path: The dish is set up in a corner of the balcony. Approx. 1m in front of the dish, directly in the signal path, there is a vertical steel support of the balcony, which goes from the ground floor through the balcony of the upper floor.

    I can "see" QO-100 just above the corner of the building. The options to move it are limited, just a bit to the back before the upper balcony obstructs the path to the satellite. Unfortunately, I have no option to mount the dish outside the balcony to avoid the steel structure. Putting a larger dish to the same place is not possible, either.

    At the first look, a 20W PA would probably solve my problem, however with an unknown amount of RF being reflected randomly. Has anybody got any experience with obstacles in the TX path with an estimation, how many dB could get lost due to this steel structure?

    Possible alternatives for a TX antenna would be a panel mounted to the steel structure itself, or a Helix pointed to the satellite close besides the steel structure. I am aware that both options have their issues. Any ideas if these alternatives would be a viable solution for 2W TX power?

    73 de Jens, HB9EKO

  • I0LYL Yes, I see the option of either mounting a separate antenna, or increasing the output power. I have to be a bit careful, though: Our rental contract says I am not allowed to MOUNT/ATTACH anything to the building, which is why the setup has to be some kind of semi-portable. I already had discussions with a previous landlord somewhere else because of a Sat dish mounted visble to the bacony railing and I prefer to avoid that experience here.

    The dish is currently on a stand inside the balcony, with limited visibility from outside, and I'd prefer it to stay that way. Maybe I could make a temporary antenna mount look like a flag pole, that seems to be generally accepted here in HB9 :-). Or hide a panel behind a Swiss flag or so.

    DD0KP I thought about something like that already, I even found a similar panel with 1 dB more: http://www.wifi-highpower.de/1…24-ghz-wifi-n-buchse.html

    Has anybody tried such a panel antenna with 2W already?

    Just in Theory: The panel antenna has a linear polarisation, i.e. I lose 3 dB. Assuming that my dish has 22dBi, (taken from a table with dish size vs. gain and rounding down), the performance of such a panel compared to an unobstructed dish is -6 dB (3 dB difference between the antennas and 3 dB for polarisation).

    The panel would then compensate a loss due to the steel structure of 6 dB. (22-19-3 dB) . So I get a better performance only if the attenuation of the steel structure is more than that. I'd like to have 6-10 dB more than my current signal for SSB. Is it a realistic that the steel structure attenuates my signal by 12-16 dB?

    Not sure if adding a 20W SG lab PA to my setup would be the safer bet. If I have a barely audible SSB signal now, an extra 10dB should be fine for a basic signal in SSB, and good results in CW.

    73 Jens, HB9EKO

  • DD0KP For that price I agree. Here they cost much more. I found the antenna on Ebay. The seller doesn't ship to Switzerland, and he can't be contacted with the "Contact Seller" function on Ebay. Looks like I have to put that on hold until I can get across the border to my parcel handling service :-(. I don't want to spend half the price of let's say the SG Lab PA more for try and error.

    73 Jens HB9EKO

  • Hard to tell but it looks like you are partially blocked from the satellite by the wall. If so, no easy solution but an alternative location for the dish. Further forward may help.


    yes. Looks like on picutere above. Is there a balcony above?

    take a look to the illumination and shadows from the sun at exact this time where the sun has the same azimuth as QO-100



  • Hello,

    G0MJW  DD4YR the picture doesn't clearly show the real direction of the antenna. When I started experimenting. I considered the visibility of the satellite already (see the thread DRO LNB for testing RX signal levels? )

    I have LOS and a reasonable RX quality (maybe not the last possible dB), that's why I got optimistic and started investing into equipment.

    I was aware from the beginning that the Fresnel zone is not clear, but there's nothing I can do about the position of the satellite, my balcony and the fact that I have to stay as stealth as possible. So now that I did my initial investment, I will just investigate further until it works. That's what ham radio is all about, isn't it? :-)

    DH2VA Looking at the setup now, I think it will be easiest to remove the concrete plate fom the bottom of the dish stand, put a box inthe corner of the balcony and move the dish temporarlily across the balcony railing. Then I have numbers in dB regarding the steel pole, and know where I am. I also hope that the situation at the borders normalize again, then it will be easier to buy components just for experimenting.

    I was hoping that someone else in the Forum might have experience with similar issues already, but it looks like I'm the first one, so I'll keep on experimenting. The results will hopefully support other OMs in similar situations.

    73 de Jens, HB9EKO

  • Just make sure you tie everything very well together. Such a dish has quite some wind load and when the thing falls off the temporary testing table your neighbors/landlords won't be exactly happy.

    Even if somebody had a similar issue already, the wavelength is only 13cm and moving the overall geometry by a quarter of it can drastically change everything. So it will be very hard to compare and even harder to simulate. You are correct though: this is what hamradio is all about, best of luck!

  • Update:

    This weekend, i had a close look at my setup again, and checked the signal path from the focal point to QO-100 +/- 10°. I then figured that there is not only the steel structure but also the balcony railing in the way. :-(

    I replaced the attenuator-style jumper from the transverter to the feed with a piece of Aircell 5. Then I started looking for a "sweet spot", shifted the dish around on the balcony with an eye on the signal strength of my own TX signal via the transponder, and found around 5-6dB. :-)

    CW QSOs are not a problem at all anymore. I even made a few successful SSB QSOs, QSO partners confirmed that my signal was weak but clearly audible. The beamwidth is now probably limited by the balcony railing on the bottom, and the upper balcony on the top. The steel pole is about 1.5m in front of the focal point. During a QSO with someone with the same TX power and dish size, I noticed around 4 dB difference in signal strength. That's apparently what disappears in the steel pillar, which is now 11-12 wavelengths from the focal point in TX direction.

    Thanks for all the hints, now I can experiment with improvements to my setup for better SSB QSOs rather than struggling to communicate at all. Lessons learnt: an obstacle in the signal path does not neccessarily make QO100 operation impossible.

    Vy 73 de Jens, HB9EKO.