Using RG-6 / U as a feedline?

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by KK4NSF, Jul 26, 2021.

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  1. KK4NSF

    KK4NSF Ham Member QRZ Page

    A few weeks ago I obtained ~600ft of new RG-6 / U coax for virtually nothing in an auction lot. So being the frugal ham that I am, I decided to give it a try in my shack. I made up a 100ft length using Home Depot W connectors, and cheap SO 239 to W adapters. I had maybe $10 in the whole 100ft line. (I checked online, and both BNC and SO 239 connectors for RG6/u are about the same price as RG-59 connectors.... so the price appears to work out fine.)

    I half expected it to work like crap, but low and behold, it seems to work as well or better than my RG-59 and RG-8X cables. Just to be sure, I flip flopped between the three several times, and checked the received signal using my spectrum analyser. I'm not seeing any noticable loss of signal. On transmit the SWR goes from ~1 to 1.5 (w the tuner settings left the same), but that is to be expected since it's 75ohm cable.

    All of that convinces me that the R-6 /U is pretty good stuff, and certainly can be used in ham applications. I don't think I'd mix it with 50 ohm cables in an array, but other than that, I think I like it. So what am I missing? Why won't my cheap R-6/U work just fine across the board?
     
    K2XT and WZ7U like this.
  2. K0UO

    K0UO Platinum Subscriber Platinum Subscriber QRZ Page

    Search the forums there's a bunch of good information on here
    I use thousands of feet of 70 ohm

    73 from,
    The K0UO " Rhombic Antenna Farm" 2 miles of wire in the Air & On the daily
     
    KK4NSF likes this.
  3. WA7ARK

    WA7ARK Ham Member QRZ Page

    The only stated reservation is that the center conductor is steel with a very thin layer of Copper plating. Some think that the plated layer is too thin for required skin effect for RF at 160/80m, meaning that the coax losses at low ham frequencies is higher than the published loss per 100ft that you read on the manufacturer's spec sheets.

    The low frequency additional loss is supposed to be a dB or two worse than it is at 10MHz and higher. I have not measured it.

    I bought a 1000ft spool and used about 500ft of it to place a HF pre-amplified receive antenna further from some noise sources that I cannot control. It seemed to work about as I expected, but I didn't notice additional loss because of the pre-amp at the antenna ;)
     
    Last edited: Jul 26, 2021
    KK4NSF likes this.
  4. SWL37632

    SWL37632 QRZ Member

    I 'get' the inner conductor skin effect loss of the cu thickness to steel issue, but I'm curious to know if there ever was some testing done to validate the additional loss at 160/80m ?
     
  5. W9IQ

    W9IQ Ham Member QRZ Page

    There are a wide variety of brands and manufacturers for RG6/U so the plating question cannot be universally answered. In any case, some work with a tracking SA or a VNA will answer the question for any particular purchase.

    The other issue to consider is that most SWR meters are built for 50 ohm coaxial systems. While the SWR50 meter right at the transmitter or in the tuner will properly indicate the SWR50 which is most meaningful for matching purposes, it will not help with antenna (load) matching or coax losses. In the case of antenna matching, using an SWR50 instrument can easily make the SWR75 worse which will unwittingly increase the coax losses.

    Put another way, on a 75 ohm system, the SWR50 meter is not reporting SWR at all.

    - Glenn W9IQ
     
    SWL37632 likes this.
  6. WA7ARK

    WA7ARK Ham Member QRZ Page

    ... let's see, project # 357 this week....

    Take the 1000' spool I have here, feed in 100W from my FT857 and measure what dribbles out the other end on various bands... I will do that after I return from flying to KPRC to meet a group of hams for lunch at the golf-course Manzanita Grill restaurant within walking distance from the airport. ;)
     
    K7JOE and SWL37632 like this.
  7. WA7ARK

    WA7ARK Ham Member QRZ Page

    When making this measurement, remember that the Vna source and input resistance is 50 Ohms, so some of the loss reported as happening in the RG-6 cable is because it is operated at elevated swr during the vna measurement. This is due to the 50/75 Ohm mismatch at both ends of the cable under test. If the loss is higher on 1.8MHz than on 20MHz, it might indicate that the skin effect is the underlying issue...
     
    SWL37632 likes this.
  8. SWL37632

    SWL37632 QRZ Member

    Your points are well taken.

    Agree that SWR50 instrument measurements are not appropriate for a 75 ohm system for this matter.

    Having a VNA available and calibrated/SOL for 75 ohm is feasible here.

    However, another method that comes to mind is actually using a pi-network TX on 160m and comparing say, copper inner conductor RG-59 to the cu-on-steel RG-6. The only blocker would be getting or building a 75 ohm load and power meter....perhaps a for-the-purpose calorimeter on the 75 ohm load.

    Your thoughts?
     
  9. SWL37632

    SWL37632 QRZ Member

    Looks like I was writing my post when you commented...

    1,000 feet would certainly give some good data....

    Thanks.
     
  10. WA7ARK

    WA7ARK Ham Member QRZ Page

    Calibrating the VNA does nothing to change the source and load impedances seen by the RG-6, so during the measurement, the cable is being operated at an elevated SWR. What would help are resistive min loss pads that convert 50 Ohms to 75 Ohms and vice versa.
     
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