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Acceptable feedline loss for satellite operations?

Discussion in 'Satellite and Space Communications' started by WA7F, Nov 4, 2019.

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  1. WA7F

    WA7F Premium Subscriber QRZ Page

    So I’m doing a quick sanity check after counting tenths of dB for awhile. I feel like a cyclist who is obsessed with losing a few grams off the bike to gain that competitive edge. This leads me to a few questions:

    1. What would you consider an acceptable feedline loss from your base station operating position to the antennas for SSB satellite work?

    2. What is your total feedline loss and antenna gain on both 2m & 70cm?

    3. How would you rate the performance of your system?

    Sure, there are some negative factors like trees, buildings, local noise levels and positives like a really nice preamp at the antenna and low noise rural locations but, at least the above questions provide some baseline. My system is still in boxes and on paper but, I am making progress on most weekends and the system will be operating soon with one of about three options. I’ll provide my calculations later but, I don’t want to detract from the above questions at this time. Thanks.

    Dave, WA7F
     
  2. KM4FVI

    KM4FVI Platinum Subscriber Platinum Subscriber QRZ Page

    Dave, I have the M2 LEO antenna array with a Yaesu G-5500 rotator package on a Rohn 25G tower, 35 FT platform. I run LMR400 to the shack (approximately 75 FT from mast mounted preamps to rigs) to a set of TS-2000 transceivers. I hardly ever turn on the mast preamps since the AOS / and loss of signal is from horizon to horizon. I use very little power to communicate through sats because you don’t want to shut down the their transceiver from transmitting too much power on your end. For example: the International Space Station (ISS) is only 249 miles from my QTH when directly overhead. I only need 25 watts out my transceiver to have a successful QSO. I suffer no reasonable amount of signal loss through my system to prevent communication with ant of the LEO birds. If I remember correctly, at a 100 watts, my PEP out the M2 LEO array is around 450 PEP; way too much power to use for LEO operations! Join AMSAT and sign up for their Email messaging system to pick other SATCOM operator’s brain before you pull the trigger on your design. Very knowledgeable operators who will share their insights with you. So to answer all your questions in a nutshell, a very simple design, and very little investment will yield great results unless you are like me ! The ARRL has several books for sale dedicated to this very subject that would make a great addition to your personal library.

    Frank
     
    Last edited: Nov 4, 2019
  3. K3RLD

    K3RLD Ham Member QRZ Page

    I'm running 50 feet of LMR240 from my pre-amps to my radios - and I can hear both 70cm and 2m downlinks from the major SSB sats with relatively no problem even without the pre-amps on (M2 antennas, Philly suburbs). The preamps do make the audio much nicer, though - and help greatly when the antennas are pointed through my house.
     
  4. WA7F

    WA7F Premium Subscriber QRZ Page

    Ok, that’s interesting info. Based on what I gathered I’ve come up with following estimations.

    Frank, it looks like you have line loss of about 1.12 dB on 2m and 2.02 on 70cm. With antenna gain of 7.06 & 11.16 dB that puts your ERP at around 239 & 500w when running 100W at the rig. At a more practical 25W the ERP is about 239 & 500W.

    Roy, with your shorter but higher loss feedline results are similar but, a bit more loss. Line loss of around 1.5 & 2.65 dB. With the same antenna system at 100W that’s 219W on 2m & 433W on 70cm. I’ve seen photos of your installation and recall that it close to house and a treed city lot.

    It sounds like Roy sees more benefit from the preamps than Frank and that makes sense given the line loss and obstacles. I know the ERP isn’t really critical in this application but, it probably gives some indication of the receive performance as well.

    I’ve got a couple of options.

    Plan-A: The M2 2mcp14 & 436cp30 antennas are 200’ from my operating position at 75’. I was fortunate enough to find a little over 400’ of AVA7-50 at a good price (average $2 per foot plus connectors) and I’d use one 200’ run for 2m and the other for 70cm. Half of my line loss is in LMR-600UF antenna loops. I’m seeing a total line loss of about .85 dB on 2m & 1.55 dB on 70cm. Using the same comparison with 100W at the that rig the ERP is 526W on 2m & 929 W on 70cm. To maintain similar ERP levels my operating power would likely be around 10 to 12W. I do have hills, trees and house and that’s why I would go to the trouble of placing the antennas at 75 feet above ground. The problem with this approach is service ability and the cost of adding another 10’ to the tower. (The shipping is more than the tower section). My HF yagi antenna would be at about 65’ on the same tower.

    Plan B: would be a second tower maybe 60’ away (same losses and gain) but, with trees, house and hills obstructing the horizon. Similar to your situation, Roy but rural so I’m not expecting much local noise that I can’t mitigate.

    Plan C: would be a new tower site about 160’ from the primary tower but use only one run of AVA7-50 and two duplexers. The loss would go up to ~2.5dB on 70 cm and the overall performance less than your system, Roy.

    Plan D: convert the existing tower to VHF/UHF only and eventually mount my 2m EME array at about 59’. The same losses as Plan A but some expected interaction between the EME and satellite antenna unless I pull the 2m CP yagi off the top and just use the EME array for satellite. Maybe this would be a better long term upgrade path for option A.

    It seems that Plans A & B are very feasible and Plan C should “should work” with greatly reduced performance.
     
    K3RLD likes this.
  5. KM4FVI

    KM4FVI Platinum Subscriber Platinum Subscriber QRZ Page

    Dave, I do have two towers. One is strictly for my SATCOM operations and my crank up tower has my Mosley yagi antennas setup. Obstacles like trees and the like will prevent AOS from horizon to horizon.

    Frank
     
  6. K0UO

    K0UO Platinum Subscriber Platinum Subscriber QRZ Page

    The answer is, as close to zero as possible
     
    AK5B and CO8TW like this.
  7. WA7F

    WA7F Premium Subscriber QRZ Page

    “At a more practical 25W the ERP is about 239 & 500W.” Oops, I botched that, meant 60W and 125W. I was also expecting that the calculator wanted dB rather than dBi so all the ERP numbers might be garbage. I guess it’s all relative anyway, it looks like I should be in good shape with line loss < 2dB. Lower would be great but, a tower that is closer to the house won’t work.

    I think I’ll stay on track with Plan-A for now. It still seems like the best option even though the investment is a bit higher.
     
    KM4FVI likes this.
  8. K3RLD

    K3RLD Ham Member QRZ Page

    Yeah, my location certainly isn't optimal - plus no tower. But I think you're on top of the situation and likely to have a very well performing station.
     
  9. WA7F

    WA7F Premium Subscriber QRZ Page

    Roy, I’ve seen a few guys with trees at their city QTH and they seem to do Ok. Some LMR-400 (or better) for Christmas would likely go a long ways to optimize the receive performance of your system. :) Last year I picked up 75’ of new Times LMR-500UF with N connectors for about 50 bucks on eBay so it’s worth watching for the right deal to come along. The LMR-600 and Heliax connectors will send you to the poorhouse in a hurry. They cost me more than the antennas.

    I’ve been watching your progression in sat ops over the past couple of years and your posts are great for mentoring sat newbies like me. Thanks for taking the time to post your experiences.
     
    K3RLD likes this.
  10. KE8AKW

    KE8AKW Ham Member QRZ Page

    If your looking for cheap Heliax connectors check Ebay. I have bought 1 5/8 hardline N connectors (for 1.2ghz) for only $30 a piece there. I have also bought 5/8 and 7/8 Heliax connectors there for very cheap prices. As for feedlines losses I'd say your good if your below 2db of coax loss. Go look up a online calculator to find out your losses in the coax. If its above 2db your either going to need better coax or a pre amp (especially above 435 mhz+)
     

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