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"Great Quality" RG6 coax

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by NM7G, Nov 24, 2020.

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  1. KA9JLM

    KA9JLM Ham Member QRZ Page

    Just think, In the day it was good for a book...
     
  2. KB1CKT

    KB1CKT Ham Member QRZ Page

    One last go at this stuff. This time I carefully calibrated the nanoVNA for a smaller frequency range, and swept over what I was interested in. I think I get 0.8 to 1.1dB per 100' for loss on the HF bands, at least for this Southwire stuff, regardless of what HF band you are on.

    Comparing to what I think is common 50ohm stuff (looking at K5R's table):
    at 28MHz, RG213 is 1.07dB/100', RG8 (or at least Beldon 8237) is 0.95, 9913 is 0.67 and LMR400 is 0.64.
    at 1.8MHz, RG213 is 0.26dB/100', RG8 is 0.23, 9913 is 0.163 and LMR400 is 0.16

    So, make of it what one will. Good stuff will work better on 160 on long runs. Good stuff will work a bit better on upper bands.

    This is for some dual shield RG6 that I bought for $40 for a 500' spool that was at Home Depot (and I think is at Lowe's too).

    I'm a bit dubious about the 50MHz data. I tried on 2m but got a strange result: the real portion would vary from 45.8 to 53.6, and the reactance from -j18.5 to -j10.5. I'll blame the setup: it was SMA (plane of calibration reference) to an SMA->BNC, BNC->UHF, UHF->F, then the cable. Maybe the connectors were not tight enough? Maybe the UHF adapter was showing its impedance bump? Not sure. Maybe later I'll try again, but I wanted to get these measurements before I started cutting up the spool. I see eBay is flooded with orange RG11 coax of no-name manufacturer, but I am not sure yet if I want to spend $150 to buy a 500' spool to see if it's any better or not. Cheap experiment I know, but...

    Measurements are again with a nanoVNA, open/short/50ohm calibration, 500' spool with other end left open circuit. Swept to find max and min input impedance, looking for zero reactance (resonance) for a couple of frequencies close to desired frequency.
    RG6_coax_loss.PNG
     
  3. N0TZU

    N0TZU Platinum Subscriber Platinum Subscriber QRZ Page

    Did you account for the mismatch loss since the RG6 is 75 ohms , not 50?
     
  4. KB1CKT

    KB1CKT Ham Member QRZ Page

    No, I followed VK1OD's page (see here).

    I believe the method works like this: The incident wave (from the VNA) does have a reflection that does not hit the coax. But the rest of it does, and travels down the coax, and is then reflected back to the start (the coax end is open circuit). By picking the appropriate frequency, the return wave, at the coax start, can either be in or out of phase (rather than some random phase angle), and thus can be either adding to or subtracting from the characteristic impedance seen looking into the coax. If the coax was lossless, then 1x the signal should be reflected back, leading to 2x the voltage--but the VNA has to deal with the initial incident wave on top of that, which complicates the matter. Then there is the fact that the coax is not actually lossless (which is what I'm trying to measure).

    So I'm afraid I don't know anything about the math on that page, nor how his calculator works. But it appears that the formula takes into account both the initial reflection (that doesn't go down the coax) and the effect of a non-matched transmission line so as to come up with an approximation of coax loss for the measurements taken.
     
  5. KB1CKT

    KB1CKT Ham Member QRZ Page

    Poking around to see if RG11 could be had, I did find some on eBay with "maybe" solid copper cores. Maybe CCS, I consider some of the ads to be "unclear" as to what they are advertising. Searching off of eBay I found Show Me Cables, which... doesn't quite sound reputable, but whatever. They claim solid copper. Their datasheet shows 0.2dB per 100' at 1MHz (not too shabby) and 0.95dB at 50MHz, which isn't greatly better than the 1.4dB I measured on Southwire RG6. Looking for independent confirmation, DSLReports indicates a similar 1.4dB loss at 50MHz for RG6 and 1dB loss for RG11, so the data seems to correlate.

    So it sounds like, I could spend the money at some point and get a nice little bump in performance if I wanted to, on 160m. I might do that, and reuse the coax for shorter runs for other antennas.
     
  6. W9GB

    W9GB Ham Member QRZ Page

  7. KB1CKT

    KB1CKT Ham Member QRZ Page

    Does that make them quality? I saw that they were a division of someone but I'm afraid I know nothing about them nor their holding company.

    I mean, I'm impressed that I can find a datasheet for their cable, which predisposes me to them, over what I might find elsewhere, but in the end, that isn't the same as "knowing" it's good quality stuff.
     
  8. W9GB

    W9GB Ham Member QRZ Page

    I have NOT had any issues with ShowMeCables,
    in fact, had a Custom Cable made (per my specs, $) a month ago.
    Due to COVID-19, they kept me updated and informed thru process.
    In the end, it was fabricated by their California mfg. operation (does military work) and I received within stated time period.
    A regular cable order (Belden, 50 foot lengths) fulfilled from Anixter’s warehouse in Chicago north suburbs.
    ==
    Other Infinite Electronics divisions — mixed results.
    L-Com
    Suffers from customer service personnel with no power to resolve issues (missing part).
    I resolved that problem by calling their US competitor, who happily provided the missing metal bracket (microwave antenna) from same Chinese fabricator/supplier.
    Pasternak
    Nice connectors, but often higher priced — than alternatives.
     
    KB1CKT likes this.
  9. KB1CKT

    KB1CKT Ham Member QRZ Page

    Thanks for the info--one just never knows, so it pays to ask around. I'll keep them in mind as I think about upgrading.
     
  10. W9GB

    W9GB Ham Member QRZ Page

    In the end — You Buy from Who you Know, and What they Consistently Deliver.
    2020 is an extremely challenging year ... like early 1970s OR 1973 Oil Embargo.
    At that time, Shortages in metals (copper) and Transportation/Logistics were also challenging.
     

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