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Author Topic: G5RV antenna question.  (Read 1133 times)
VK2TXB
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Posts: 28




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« on: July 28, 2005, 05:11:26 AM »

Hi guys,
I am looking into buying either a windom or an antennasmore.com's G5RV 102' antenna. The price difference is quite alot BUT I have been looking through my ARRL antenna book and it looks quite easy to build a G5RV, 102' of wire and 34' of 75 ohm twinlead. Can anyone tell me if there is much difference firstly between the 2 antennas,and secondly is it worth my while to build the G5 myseld and save 40 odd bucks? Also, being an aussie what is 75 ohm twinlead? I know of 75 ohm coax, 300 ohm twinlead, but not sure about 75 ohm twinlead. Not much basics about it either in the arrl book. Can I use another more common found 75 ohm lead? Thanks for reading, and hope you can give me some help, and save me some $$$$
Cheers
Andrew
VK2TXB
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WB2WIK
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Posts: 20574




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« Reply #1 on: July 28, 2005, 08:19:53 AM »

I don't know where you got the "75 Ohm twinlead" idea, but it's wrong.

The G5RV is made from 102' of wire and 34'10" of 450 Ohm ladder line (or "window line"), or, alternatively, 102' of wire and 33'0" of 300 Ohm twin lead.  The 450 Ohm ladder line is slightly superior.

See the ARRL Antenna Book, 20th Edition, page 7-7.

As for "build vs. buy," of course it's probably cheaper to build one.  The only tricky parts are:

-Joining the ladder line/twin lead to the doublet elements in a manner that is strong mechanically and doesn't stress the transmission line, while making a good, permanent electrical connection.

-Terminating the ladder line/twin lead, to join it to the coaxial cable which makes up the rest of the transmission line to the station.  What I usually do is use a simple 1:1 current balun at this point, one of the pre-packaged ones that has wire terminals and an SO-239 coaxial connector, thus making this transition very easy.  There are other ways.

WB2WIK/6
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VK2TXB
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Posts: 28




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« Reply #2 on: July 28, 2005, 10:57:21 AM »

Hey thanks for info!
I got the 75 ohm twin lead from the arrl 19th edition book , page 7-5!!! I thought that was weird as well, you know an american thing or something! What I am wondering, is what would be the best all round wire antenna I can build to cover 80-6 mts? I know this has been covered, I read all 62 pages on this forum section!! (and I am seeing double now!), BUT here it goes. The question of best all round wire antenna for 10-80 mts has been asked, BUT really, no one has really answered it basically! I know alot of newbies would like just a basic practical plan to build something and the answers they get half the time is never a practical, hands on, this is how you do it, answer.  I don't mean to sound cynical but I have gone through alot of questions people have asked and alot have been given good answers, others just try and sound like they work for NASA! As for the ARRL antenna book, it is pretty good BUT they need to put some ground level examples for people to build, to enjoy achieving a working antenna project, instead of being just technical references. They just need a balance I believe, technical info plus REAL projects you can build to enjoy. I am sure there are many others like me who feel the same. But, I appreciate anyone who takes time to respond to my question, cheers
Andrew
VK2TXB
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WB2WIK
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Posts: 20574




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« Reply #3 on: July 28, 2005, 01:26:44 PM »

>RE: G5RV antenna question.  Reply  
by VK2TXB on July 28, 2005  Mail this to a friend!  
Hey thanks for info!
I got the 75 ohm twin lead from the arrl 19th edition book , page 7-5!!! I thought that was weird as well, you know an american thing or something! What I am wondering, is what would be the best all round wire antenna I can build to cover 80-6 mts?<

::Probably the best practical wire antenna for 80-6m that can be reasonably homebrewed and installed without access to special equipment or materials is an 80m horizontal loop antenna, 65 feet per side arranged as a square loop and installed parallel to earth and as high above earth as possible.  A good height would be 65 feet above ground; if that cannot be achieved, then any height above 33 feet works.

Such an antenna, trimmed for resonance, will be resonant at:

3.6  MHz
7.2  MHz
10.8 MHz (close enough for 30m work)
14.4 MHz (close enough for 20m work)
18.0 MHz (close enough for 17m work)
21.6 MHz (close enough for 15m work)
25.2 MHz (close enough for 12m work)
28.8 MHz
50.4 MHz

As such, this will be an easy antenna to "match" on all these bands, and probably won't even require a tuner.  MUCH nicer in that regard than a G5RV, not to mention higher performance.

WB2WIK/6
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N3BIF
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Posts: 1190




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« Reply #4 on: July 28, 2005, 01:49:28 PM »

 75 ohm twin lead does/did exist, but I haven't a clue where to find it. Pioneering Tv cable companies used it quite extensivly back when.....
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WB6BYU
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Posts: 13171




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« Reply #5 on: July 28, 2005, 02:05:02 PM »

G'day Andrew -

Nobody will tell you what the "BEST" antenna because there
is no such thing.  There are many good antennas, including
the loop, multiple dipoles on a single feedline, random
wires fed with a tuner, Sterba curtains, etc.

Everybody has different limitations and preferences.  Do
you want to work DX on 40 and 80 metres, or just around
Oz?  How big is your garden?  What supports do you have
available?  What types of antennas will your spouse /
neighbors / local Council permit?  How much are you willing
to spend?

The 80m loop that Steve WIK suggested is a good choice
for a simple antenna if you have 20m x 20m of space
available.  A better antenna, of course, would be a
80m tall tower filled with beam antennas for each band.
But neither would be practical for someone in a flat or
a caravan park.


Now that you've finished reading this forum, I suggest
you have a look at W4RNL's web pages:

www.cebik.com/radio.html

He provides radiation patterns for many different
antennas, including the G5RV and horizontal loops.  You
can get a sense of which will work for local contacts
and which would be better for DX.  Also, give some thought
to what directions you want to work:  if you have one
favored direction, then you want to make sure that the
antenna always radiates in that direction.  But if you
are willing to work in any direction, other designs that
change the direction on different bands might work better.
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KG6WLS
Member

Posts: 507




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« Reply #6 on: July 28, 2005, 02:39:53 PM »

I'm sure at some point in time...someone will mention a fan dipole Smiley
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VK2TXB
Member

Posts: 28




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« Reply #7 on: July 28, 2005, 10:19:57 PM »

Thanks guys!!
I really appreciate all the positive feedback, and PRACTICAL advice! I really like the Eham website, but sometimes people seem real grouchy,,ha ha.. hey I didn't mention CW code OR CB radio!! (mind you I liked CB radio when I was younger, sparked my interest in ham).
I have looked at the fan dipole as well and that looks pretty cool, a slight eyesore for the XYL, but pretty interesting. Oh, interesting question, I had a 10 metre dipole that had a commercial 1:1 balun on it and I wanted to add a 6 mtr dipole on same setup, BUT I couldn't get 6 to behave at all! Could the balun have anything to do with that? I think it for up to 30mhz, got it from AES in vegas. Also, as to what is best, I think I speak for most new hams, the best bang for buck that is a proven workhorse and can be built by ones self and doesn't require you to be a nuclear scientist! With the 80 mtr loop idea, how do you trim it for resonence? ie do you do it on 80 mtrs firstly, and then hope other bands work? Is it basically just trimming or adding length? I have a 259B which should help. I think I can run the loop idea, as I have trees and space, now how do I get up those trees??? Mmmmmm...I think the neighbours kids like to climb......
Cheers
Andrew
VK2TXB
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WB2WIK
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Posts: 20574




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« Reply #8 on: July 29, 2005, 07:44:00 AM »

>RE: G5RV antenna question.  Reply  
by VK2TXB on July 28, 2005  Mail this to a friend!  

BUT I couldn't get 6 to behave at all! Could the balun have anything to do with that?<

::Of course it does.  You can't use an HF balun on six meters; the commercial ones I've tested roll off about 40 MHz.

>With the 80 mtr loop idea, how do you trim it for resonence? ie do you do it on 80 mtrs firstly, and then hope other bands work?<

::Yes, you do.  No "hoping" involved; any loop is resonant on all harmonics, both odd and even.

>Is it basically just trimming or adding length? I have a 259B which should help. I think I can run the loop idea, as I have trees and space, now how do I get up those trees???<

::You'll probably find the feedpoint impedance of the loop is higher than 50 Ohms, more like 100 Ohms or so, although this will vary with height above ground and also with frequency.  No big deal.  I've often just used 75 Ohm coax and fed them directly.  If you have a decent antenna tuner, you might want to use 300 Ohm twin lead to feed the loop and the balanced terminals of the tuner, and just use it.  There will be a mismatch at the feedpoint, but the tuner will tune it out and the line loss will still be very low.  It's easy to get up into the trees.  I usually tie a lightweight nylon monofilament fishing line to a heavy sinker and throw it up over the tree.  Use that line to pull up a heavier line (like Dacron rope), and use that line to hoist up an insulator with the loop wire strung through it.  Don't pull until the insulator is up against a tree limb, leave the insulator a few feet away from the tree, tie off the rope and go do the next corner.  Using this approach, if I have four suitable trees for the loop, the whole antenna is up and operating in less than an hour.

WB2WIK/6  
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VK2TXB
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Posts: 28




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« Reply #9 on: July 29, 2005, 10:01:28 AM »

Hey Steve,
Thanks for the info, really appreciate it mate! Okay, I am mentally picturing this new 80 mtr loop, and things are looking good. I only have a MFJ 945E tuner that doesn't work with twin line (well I think it doesn't?), so if I was going to use 75ohm coax, what is the difference of using 75ohm compared to some 213 I have? Also, can it be normal rat shack 75 ohm tv coax or a specific type for this loop? Thanks for your help guys, and especially Steve WIK, I appreciate it!
Cheers
Andrew
VK2TXB
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WB6BYU
Member

Posts: 13171




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« Reply #10 on: July 29, 2005, 12:51:32 PM »

Hi Andrew,

I've had good luck using a 4 : 1 balun on the antenna
with the coax running from there to the shack.  You can
also use 50 or 75 ohm coax connected directly to the
antenna, or 300 ohm twinlead or other open wire line.
I've been known to use "zip cord" (parallel-conductor
appliance power cord) or just two "sort-of parallel"
wires running through the attic and dropping through
the ceiling over the operating desk.

Look on the back of your tuner - I suspect you will see
a pair of binding posts (red?) for balanced line.  Put
the antenna switch in the "single wire" position and
connect a short piece of wire from the "single wire"
terminal to the nearest "balanced line" terminal to
enable the internal balun.

My usual method of putting up a loop is to lay the
wire out on the ground as near as I can.  Then I use
electric fence insulators - the type that you can slip
over the wire, rather than threading the wire through
it.  Put the insulator over the wire at a corner of the
loop, tie a rope to it and toss the rope over a tree
branch.  (A slingshot/catapult with a 20g fishing weight
and a fishing reel may be big here.)  Tie the wire off
with good durable rope over the branch and down to ground
level - this permits repair/adjustment later without
having to climb the trees.

The resonant frequency is set by the length of the
wire (plus a number of other factors.)  84m is a good
starting value.  If you are going to be using a tuner,
then exact resonance isn't important anyway - just get
it close.

good luck!

- Dale WB6BYU  ex-VK2DJW, ZL3AGH
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VK2TXB
Member

Posts: 28




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« Reply #11 on: July 29, 2005, 10:03:30 PM »

Hey Dale,
Thanks for info! I will be back in Vegas around Oct so I will stop into AES and grab a 4:1 balun off them. I plan on using 213 that I have and my tuner is only a crappy MFJ 945E, there is no binding posts on back, only 2 x coax plugs and a a ground wingnut. Would you recommend insuated wire, compared to bare coax? This setup will be for my place up in Vancouver BC and I have lots of trees up there, but also weather extremes. (lot's of rain!). Do you believe this loop will be better, or at least equal, to a commercial bought G5RV or Carolina Windom? Also, if I use the 4:1 balun (a commercial bought one) will loop work with 6mtrs, or will balun not allow for this? (I know I was told previously my 6mtr troubles were because of a HF 1:1 balun, BUT you have to ask questions to gain knowledge!)
Thanks for info guys!!
Cheers
Andrew
VK2TXB  
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WW5AA
Member

Posts: 2088




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« Reply #12 on: August 02, 2005, 11:27:05 AM »

Andrew,

I have a 294’ horizontal loop at 20’ that works 80 thru 6 meters, 160 meters when switched to as indicated below. The antenna is 75 ohm coax (Yes radio check stuff will work fine) in a Square configuration. I feed it with 50 ohm coax, about 55’ I use a current balun at the feed point (10- 6”turns of the feed line carefully turned so the turns do not cross each other). I use a IC-500 AT tuner since I sometimes need 1kw calling a net on 80 when the band is dead. The 500 AT doesn’t tune 6 meters, but the tuner in the Icom 746 does just fine. This antenna is far superior to a full size dipole or G5RV. Don’t get me wrong the G5RV is a good compromise antenna is space is limited. I have a coupler at the operating point via a coax switch that couple the antenna to a 155 ‘ buried counterpoise for operation on 160 meters.  Good luck, hope this helps.

73 de Lindy
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