Using 75 ohm RG6 Quad???

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by KI6BCX, Feb 18, 2011.

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  1. KI6BCX

    KI6BCX Ham Member QRZ Page

    Hello Friends,

    I found out I can stick it to the HOA and hang a multi-band dipole, but it will take a +/- 400 foot coax run to a hillside outside their boundary. I can fairly easily lay a PVC conduit line to the site, but the cost of RG8/U is presently out of my budget.

    I've heard about Emcoms making use of 75 ohm TV coax during Katrina and in Haiti, but was wondering just how it would work for everyday use?

    I was thinking of shorting the cables to provide more conductor to handle my max 100 watts and use 1:1 Baluns at each end to connect to the 50 ohm SO-239 connections at the tuner and antenna.

    So, will this affordable way work at 400'? Will it get me on the air for 20-80 meters until I can afford the RG8/U? Or am I way off the charts on this idea?

    Thanks to all in advance!

    P.S.; And yes, I've talked to the hillside owner and have full permission for the antenna. He also likes the idea of beating the HOA.
  2. VK1OD

    VK1OD Ham Member QRZ Page

    If the line is operated at low VSWR, losses are probably manageable. For example, with a 50 ohm load (so VSWR=1.5), loss is about 50% at 20m, better at lower frequencies.

    You said a multiband dipole. If that means operating the coax at high or worse VSWR, the picture gets worse.

    Paralleling the coax reduces Zo, but matched line loss is the same.

    I would make sure that the coax used uses a solid copper inner conductor for best loss below 10MHz.

    Last edited: Feb 18, 2011
  3. KI6BCX

    KI6BCX Ham Member QRZ Page

    I have the option of buying a 500' roll of twin lead as opposed to the 1000' run of single. Shounds like I should go for it. And I'll check on the inner conductor.

    I was thinking about going with a 10m-80m G5RV. I'm open to alternitive suggestions and comments.


  4. VK1OD

    VK1OD Ham Member QRZ Page

    This is not very clear to me. Twin lead can mean so many things. Is it twin RG6? If so, why is it more suitable than single RG6?

    G5RV with the hybrid open wire / coax feed arrangement operates the coax at very high VSWR on some bands, and I have already suggested that will have high losses. Have you researched the G5RV at all?

    If you are to use RG6 as you proposed, you will achieve higher system efficiency if you operate at low VSWR, so you should consider multiband antennas that have low VSWR on each band (eg fan dipoles, trapped antennas etc).

    You will probably still need an ATU to mop up impedance transformation at the tx end of the line.

  5. VK1OD

    VK1OD Ham Member QRZ Page

    I should have mentioned that under the same conditions, efficiency of RG213 (or RG8) would be around 52%, about 1.5% higher. That might temper your desire to purchase RG213.

  6. K9KJM

    K9KJM Ham Member QRZ Page

    About the best low cost way to run 400 feet would be to locate some surplus cable TV "Trunk line" Or "feeder" hardline coax of at least 1/2", Or better 3/4" diameter. Find a local cable TV construction crew, Offer them a bag of donuts and you should be able to get lots of it. (Called spool ends) While you are at it, Ask for a few common cable TV "Pin" type connectors... Ask for the ones the splicer cut the pins too short on.... They adapt directly to a PL 259 type connector. (Usually you will have much better luck finding a construction crew, Going through the cable TV front office sometimes does not work so well) This "hardline" is smooth sided, 75 ohm, Aluminum, And being filled with foam dielectric, Has close to zero scrap value. Most companies are very happy just to get rid of it. IF you get really lucky you might even find some that is "jacketed" for direct burial. Buy the crew a case of soda or beer besides the donuts if you do!
  7. W0BTU

    W0BTU Ham Member QRZ Page
  8. KI6BCX

    KI6BCX Ham Member QRZ Page

    I would like to address this myth right now... Here in CA, all bringing donuts and sodas to cable construction crews will get you is fat crews!

    OK, such tactics may work in the midwest and eastern states where such crews are friendlier, union and one dosen't need advanced foreign language skills to explain your need. But here in SoCal all you get are funny looks from the foreman and angry looks from the crew who long prior called dibbs on any scrap they can possibly salvage the slightest amount of copper or aluminum from!

    If anyone has scored in this manner, my hats off to them!

  9. KI6BCX

    KI6BCX Ham Member QRZ Page

    Sorry for the confusion... Some call it "shotgun" because it's side by side. The rolls I've seen at the mega store all say "Twin" so that's what I called it.

  10. KM1H

    KM1H Ham Member QRZ Page

    I also use CATV hardline for TX runs up to 450' and out to 750' for Beverages for receive only. Even on 6M a 450' run of 3/4" and larger has very low attenuation to a matced antenna. With a typical 10M G5RV VSWR in the 2:1 range the total loss should be under 2dB 24/7 unlike open wire line or transmitting twin lead. While the loss will be negligible on RX unless you are working at the noise level it could impact your TX being good copy at times. Heck today the MUF is above 30MHz and 10M is wide open to SA and some Pacific islands; signals are pounding in.

  11. VK1OD

    VK1OD Ham Member QRZ Page

    A back of the envelope analysis of the G5RV on 10m...

    The G5RV is a three halve waves dipole at 20m, so on 10m it is about three full wavelengths. Fed in the centre, it is fed at a voltage maximum, and at resonance you can expect a feed point impedance of somewhere around 2000+j0Ω.

    If you use the so called open wire matching section, it is an electrical half wave on 20m, so a full wavelength on 10m and the impedance looking into it is about the same at at the dipole centre, lets call it 2000+j0Ω.

    If you explore the loss in 400' of Belden 1694A (RG6 with solid copper centre conductor) using TLLC with this load, it is 12dB or an efficiency (Pout/Pin) of 6.4%.

    Same scenario for Commscope P3-750 JCA (a 3/4" CATV hardline), the loss is 6.3dB or 24% efficiency.

    You can achieve much higher efficiency if you choose an antenna with a low line VSWR.

  12. W0BTU

    W0BTU Ham Member QRZ Page

    I'm glad you posted this, Owen, because I kind of half-suggested a G5RV to a friend who lives in a small city lot, as an option he might consider. I only looked at the G5RV design one time, and didn't know this.

    I love your back-of-the-envelope analyses. :)
  13. VK1OD

    VK1OD Ham Member QRZ Page

    Mike, it happens that 10m is one of the worst of the original 5 bands claimed of the G5RV with hybrid feed. With shorter feed line, the loss is manageable (about 2dB for 20' of RG6)... but then there is the pattern to accept.

  14. KM1H

    KM1H Ham Member QRZ Page

    You have to watch those envelope squiggles real close to catch the errors, misinterpetation, half finished, or whatever. Ive done similar on barroom napkins when I still visited those places.[​IMG]

    The G5RV was initially developed for open wire line where VSWR wasnt considered important enough to worry about. Along comes coax and something new had to be done which is the so called vertical stub. Next a few smart fellows come along with a ferrite balun match using coax cable which seems to do the job in pre computer and pre SS rig days. Then later W1JR comes along and publishes someones idea of a toroid balun match with flipping half the winding. Then later on thats dismissed as wishful thinking.....and the beat goes on.

    The best I could do with any ferrite intervention was about 2.5:1 VSWR on 10M which was OK with a 75A4, CE-100V, and NCL-2000 amp and about 60' of fresh RG-8 foam. This was in the early 80's and I had just moved and needed to get something up that was simple and easy. At about 60' high between pine trees and otherwise open space it did OK on 80-10. With nothing to compare against, a gut feeling said it was a bit down from a dipole on all bands but 20 where it cracked its share of pileups. Soon after I built a 4 el 10M monobander and had it on a chimney mount at about 15' high and there was no comparison, the yagi was like turning on a switch. I still have that radio setup BTW (owned since 1965) and is used for nostalgia days when the mood hits.

  15. K8JD

    K8JD Ham Member QRZ Page

    Most of my HF antennas are in the woods behind my house and require LONG runs of feedline. I bought RG8X in bulk to save costs. My runs are up to 350 ft.
    The best thing to improve efficiency is design your antennas for minimum reflected power, trimmed and tuned for each band!
    My latest is a FAN dipole with wires for 20 and 30M elements. This way I have two bands with one feedline.
    You should consider a fan dipole with elements for the bands you intend to operate on.
    This approach uses more wire but sure BEATS a single wire dipole with high SWR on all bands !!! Using coax line is a given since Twin lead and ladder line is not going to work when buried in the ground.
    By the way, This location for my antennas is practically noise free compared to when I had antennas between the houses.
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