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Author Topic: G5RV Jr., Balun, and Antenna tuner  (Read 6024 times)
KC9LTO
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Posts: 24




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« on: April 16, 2008, 06:20:41 AM »

Hello all.  I've decided to order MfJ's G5RV Jr. (MFJ-1778M)(52 ft. for 40-10m).  The manual I downloaded talked about using a balun between the ladder line and the coax cable leading to the antenna tuner (it mentioned the MFJ-915 RF Isolator or gave directions for winding your own balun).  I have an MFJ-949B (an older tuner) with an included 4:1 balun.  My question--do I need the Balun that the G5RV manual talks about?  Those who use the G5RV Jr.--what's your experience been?

Thanks,
Ann
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WW5AA
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Posts: 2086




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« Reply #1 on: April 16, 2008, 08:20:33 AM »

Ann,

Wind a coaxial balun (10-6" turns), close wind so that turns do not cross. The antenna you described will not work well with the 4:1 balun in the tuner. There are many better options than this antenna if you have not ordered it yet. Have fun.

73 de Lindy
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WA7NCL
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Posts: 625




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« Reply #2 on: April 16, 2008, 08:31:24 AM »

If you have the tuner with a balance output, save yourself the money on the g5RV and use ladder line to a dipole cut to half wave on your lowest operating frequency.  If will most likely tune on all bands above the lowest using the balanced output of your tuner.  It won't cost any more than the G5RV and it will work better.

If you insist on paying MFJ, then I think they have some doublet antennas, with ladder line and wire and insulators you can buy.  Go with that.

The problem with the G5RV is the feed line, balun losses and the common mode current problems due to offcenter feed.  Why the G5RV has developed an almost mystical cult following is beyond me.
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K7KBN
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« Reply #3 on: April 16, 2008, 08:40:16 AM »

I've never seen a G5RV with OCF.  Is this a new development in this "antenna" or what?
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73
Pat K7KBN
CWO4 USNR Ret.
WW5AA
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Posts: 2086




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« Reply #4 on: April 16, 2008, 09:31:07 AM »

"I've never seen a G5RV with OCF. Is this a new development in this "antenna" or what?"

Hard to tell. I don't know anyone running a real G5RV. As I have said before, poor Mr Varney is turning over in his grave. The real G5RV was 102' flat top, not inverted "V", fed with 34' of 525 Ohm parallel feeders and 72 Ohm coax. It was designed as a 20 meter antenna that could be used on other bands when nothing else was available.

73 de Lindy
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KC9LTO
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Posts: 24




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« Reply #5 on: April 16, 2008, 10:27:55 AM »

Thanks for the information.  Lindy--what you described sounds like the instructions given for the "homemade" balun in the instructions from MFJ.  

I know this isn't a "true" G5RV, but it fits my needs for an antenna for a smaller amount of space.  It's setup is similar to that of various QST articles discussing dipoles made with varying lengths of antenna with ladder line.  

Any other suggestions would be appreciated.
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K8JHR
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Posts: 29




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« Reply #6 on: April 29, 2008, 09:20:10 PM »

I believe the balun is important to the proper working of a G5RV antenna.  The window line is radiates and the isolation balun is designed to terminate the radiator and separate it from the coax feed line.  You cannot use the balun on the tuner as it moves the termination point inside the shack - look for stray RF and you will also un-tune the antenna.  

Just MY take.   Good luck.  //  K8JHR  //
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WB6BYU
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« Reply #7 on: April 29, 2008, 10:23:14 PM »

There is nothing wrong with calling the half-sized version a G5RV, as
long as you clarify the length.  We don't call a yagi, Moxon or Windom
different names when we build them for different bands.

If you go back and actually read the articles writen by Lou Varney, G5RV
himself (several are available on the internet) his prefered feed system
was balanced line all the way to the shack.  This generally is the lowest
loss option and allows the antenna to be used efficiently over a wider
range of frequecies.  The common feed system with a mix of ladder
line and coax gives a reasonable match on some frequencies, depending
on how much loss you are willing to tolerate.  It was originally designed
to be used with 75 to 90 ohm coax, which still may be a better option.
Here is a good analysis of loss vs. frequency for a full sized G5RV:

http://www.vk1od.net/G5RV/index.htm

Doubling the frequencies will give you a rough estimate of the losses
of the half-sized version, but in practice you'll find the coax losses
will be a bit higher (feedline loss increases with frequency) and the
tuner losses will drop a bit with reasonable-sized components.  He
also has a page on optimizing the match for your specific components
and frequencies.  Figure 12 of this article shows the losses with balanced
line all the way to the shack.  (The problems with radiation from balanced
lines in the shack is often overestimated.  I've seen more problems
with RF from coax feed than from balanced line.

There are a number of pages on G5RV on W4RNL's web site, too, if you
can get access to it.  (I haven't managed yet.)

If you don't use a balun at the bottom of the matching section, you can
get RF flowing on the outside of the coax (and back into your shack
if you aren't careful.)  The amount of RF will depend on the length of
the coax, how  your station is grounded, etc.  Perhaps a worse problem
is when the coax is laying on the ground:  the lossy dirt will attenuate
the RF before it reaches the shack, but unfortunately that is simply
wasted power that doesn't get radiated.


So if you run balanced line all the way to the shack you can use the
balun on the back of your antenna tuner.  (It may work better to use a
1 : 1 balun on some bands, depending on the feedline length.)  If you
use coax feed you need some sort of balun/choke at the junction
between the coax and twinlead.  (Well, you don't absolutely NEED them,
as you probably can make contacts without them, but the results will
be more predictable if you do.)
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KD8CGF
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Posts: 39




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« Reply #8 on: April 29, 2008, 11:08:46 PM »

    The MFJ-1778M manual states "The best balun for this antenna is an air-core choke balun."   Unfortunately there is no illustration.  Check this out:  http://www.qsl.net/ta1dx/amator/Rfchoke.jpg   This type of balun is trivially easy to fashion, the manual makes it sound more difficult than it is.
    I don't know the details of the MFJ-949B, but I have an MFJ 949-E.  It does have its own 4:1 balun, but this comes into play only when balanced line (the ladder line) is connected directly to the tuner.  If you are running coax from the end of your MFJ-1778M to your tuner, this is not a balanced line connection.   My MFJ-949E has a separate connection for coax, and this connection bypasses the internal balun completely.  I imagine the 949B is similar.
    I have been using a Van Gorden Shorty All-Bander, which is similar to a G5RV Jr. & roughly the same size.  It came with 100 feet of 450 ohm ladder line of which I use about 50.   The ladder line terminates directly into my tuner through a hole in a basement wall.  I have not had trouble with RFI as long as my XCVR has been properly tuned into a dummy load before begin connected to the antenna.  I tried the Van Gorden without using a dummy load first & the results were not satisfactory.  I also use a 40M dipole connected to the tuner by a run of  coax through the tuner's coax connection -- for this antenna I use an air core choke balun almost exactly as illustrated above, hanging high up in the air at the center of the 40M dipole.  I did try to use the 40M dipole without the choke balun & could not get it to tune properly, but with the balun tuning was easy.  
   "I've decided to order MfJ's G5RV Jr. (MFJ-1778M)(52 ft. for 40-10m). The manual I downloaded talked about using a balun between the ladder line and the coax cable leading to the antenna tuner (it mentioned the MFJ-915 RF Isolator or gave directions for winding your own balun). I have an MFJ-949B (an older tuner) with an included 4:1 balun. My question--do I need the Balun that the G5RV manual talks about? Those who use the G5RV Jr.--what's your experience been? "
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N4KC
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Posts: 288


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« Reply #9 on: May 01, 2008, 01:54:32 PM »

Ann, as previously noted, the MFJ G5RV as designed is only a 40-10-meter antenna if you are using a matchbox/tuner more robust than most rig internal tuners.  I also like the suggestions that you simply make the antenna longer than 1/2 wavelength on the lowest band you plan to use it on, feed it in the middle (both sides must be equal to be a true balanced antenna) with ladder or window line, and use the tuner to match it to your rig. That WOULD be a good all-band antenna.  Get it as high in the sky as you can and make sure the window line stays at a 90-degree angle from the antenna as far from the feedpoint as possible.  Except for the length of window line from the balun/coax to the feedpoint, and the length of the antenna itself, this is no different from what most call a G5RV (though, as also noted, that was not Mr. Varney's aerial).

There are several ways to get the balanced ladder/window feedline into the shack, including running it to a 4-to-1 or 1-to-1 current balun (there are arguments for and against either ratio), then on to the unbalanced output of your tuner using the shortest run possible of coax.  See what I mean about this being very similar to the "G5RV."  See my web site article on my loop antenna for another method (www.n4kc.com) of getting balanced line into the house.

The G5RV as typically sold--even the shortened version--can be a useful antenna on several bands.  I bought the Wireman's ZS6BKW-version "short G5RV" kit for less than I could have bought the wire, feedline, and insulators.  If the window line is measured carefully, you don't even need a balun at the changeover to coax, but it does not hurt to have one and may help.  I have no stray RF on the coax shield and the silly thing is not bad at all on 40 and 20 meters.  It's okay on 17, too.  But forget 30, 15 and most of 80/75 using the internal tuner.  I have better choices on all bands but it's surprising how often the G5RV is slightly better under certain conditions.

Antennas.  You gotta love 'em!

Don N4KC
www.n4kc.com
www.n4kc.blogspot.com
 
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KC9LTO
Member

Posts: 24




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« Reply #10 on: May 06, 2008, 01:16:20 PM »

Thanks for all the info.  Sorry I haven't replied; I just checked back into the forum after many weeks away.  I did order the G5RV Jr. from MFJ, and have received it.  Have yet to put it up because of busyness, no voice (on one weekend--takes communication to put up an antenna with another person), and weather.  Maybe June...  

A lot to think about with the Balun.  The person who recommended this sort of antenna to me has the same sort of antenna and does not have a balun, and says I don't need one; his works fine, he says.

I'll have to keep reading through this info... (or give it a try without--can always add one).  Now, how to get the coax into the house... (but that's another forum).

Thanks!

Ann
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KJ4EOZ
Member

Posts: 1




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« Reply #11 on: July 07, 2009, 02:42:10 AM »

You must use a balun
to balance your dipole

By feeding the antenna with properly grounded coax you get the benefit of a shielded lead-in, which will reduce electrical noise pickup from near your house. Its usually signal/noise ratio, not just signal strength, which is important.
This is true only if everything is connected properly. You cannot just connect co-ax to a dipole and then to your receiver and expect it to work properly. A dipole is a balanced device, co-ax is an unbalanced feeder and the inputs on most modern receivers are unbalanced. You cannot just mix balanced and unbalanced configurations without a balun.
Consider that the signal induced in one element of the dipole is 180-degrees out-of-phase with the signal induced in the other element. A balun is required to reverse the phase of one of the signals so that they are additive. Assuming the use of twin-feeder line from the dipole to the balun it follows that any noise (or signals) picked-up by the feeder will be in-phase between the two lines and will, thus, be effectively cancelled when the phase of one of the signals is reversed.
If you do not use a balun you end up with just a random-wire antenna because the signal from one element of the dipole is just sent straight to ground and contributes nothing. And if you use co-ax from the dipole to the receiver without using a balun you can get significant capacitive losses.
Many SWLs find that a "dipole" often gives no improvement or even inferior results. The reason is usually because a balun has not been used.
There is a great deal of misunderstanding concerning the function of a balun with many regarding it as just an impedance-matching device. A balun may well match impedance but it does not have to - it can easily have an impedance ratio of 1:1.
What is important is that it matches balanced to unbalanced lines. After all, balun is short for "balanced-to-unbalanced"!
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K7KBN
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Posts: 2838




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« Reply #12 on: July 07, 2009, 12:19:47 PM »

Many of us have used dipoles for a half-century and more, feeding them with coax and without baluns, and have done quite well.  I got 40 meter DXCC with just such a dipole, fed with surplus RG-8/U.

The radiation pattern of this antenna may not have been quite as shown in the ARRL Antenna Book, and I don't even know (or care) what the SWR might have been.  In the early 1960s, nobody but the very wealthy had anything that would measure this mysterious "SWR" thing, but as long as our transmitters loaded properly, it didn't matter. Probably had a ton of loss in the coax, but the point is, we made contacts and had fun.

You can still have fun today, and without baluns.  They have their place, but in many cases, they aren't absolutely necessary.
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73
Pat K7KBN
CWO4 USNR Ret.
K1BXI
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Posts: 812




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« Reply #13 on: July 10, 2009, 12:22:02 PM »

"This is true only if everything is connected properly. You cannot just connect co-ax to a dipole and then to your receiver and expect it to work properly."

Since when?............as long as the outside of the braid of the coax (that 3rd leg) shows a high impedance, what's the problem.

John
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WX7G
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Posts: 6328




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« Reply #14 on: July 10, 2009, 01:34:59 PM »

Dipoles, coax and baluns:

A dipole is two wire antenna. Then fed by a coaxial cable and no balun you have a three wire antenna: The dipole right half, the dipole left half, and the coaxial cable shield. We can call this a "tripole."

Let's say the shield is connected to the left side of the dipole. And let's say the shield presents the same impedance that the dipole left half does. The feedline is orthogonal to the dipole wires. In free space this would be 36 ohms with the feedline being 1/4 wavelength. Remember that the coax inner conductor and inner shield current are exactly equal. Now let's say 1 amp flows in the dipole right half. And 1/2 amp flows in the dipole left half. And 1/2 amp flows on the outside of the coax shield. The left side of the three wire antenna looks like 18 ohms (36 ohms || 36 ohms) while the right side looks like 36 ohms. The coax is now driving 18 + 26 = 54 ohms. Not bad. The radiation pattern is not the classic 'dipole' pattern but it is radiating.

Now let's bring our "tripole" down to ground and hook it up. If the coax is the right length there will be shield current roughly equal to one-half the dipole current. The shield radiates as much as 1/4 of the total antenna energy. So what? Now let's change the coax length to where the shield presents a high impedance at the dipole. There is very little shield current. A balun would do little to change the currents.

Conclusion:
If one wants a clean classic dipole pattern and to reduce the possibility of shield radiation causing RFI (because it is closer to sensitive devices), go ahead and use a 1:1 current balun. Otherwise go without a balun as everyone did successfully for years.
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