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Author Topic: GVRV and the Polyphaser  (Read 1039 times)
K1BXI
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« on: June 23, 2008, 08:50:56 PM »

WB6BYU............your absolutely right about the impedance of the twin lead. Try using 300 ohm (the original G5RV) and see if you get a better match on 80. I find it to be around 25-30 ohms at the end.

John.....K1BXI
 
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WALTERB
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« Reply #1 on: June 23, 2008, 10:34:45 AM »

Hi,

I just purchased a True-Talk G5RV. It has 30 feet ladder line and 70 feet of coax. My understanding is that the length of this is critical to it working correctly. Anybody have any suggestions as to where to add a polyphaser lightning arrestor to the antenna? I assume this should go on before it enters the shack? So should I spool up 50 feet of the coax or should I just put the polyphaser on inside the shack and attach it to a grounded cold water pipe? Something tells me this is not a good idea. Basically from the antenna at ~30 feet to the radio is about 75 feet. and in order to add the polyphaser and keep the 70 feet of coax means I need to wind it up or something. thanks  
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KM5Z
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« Reply #2 on: June 23, 2008, 10:49:46 AM »

The ladder line forms a partial match for the antenna and also radiates.

You'll need maybe 10 turns of coax for a choke at the point where you connect to the ladder line. This can be loose turns or you can roll it neatly on a 5" or 6" PVC form (I've done it both ways - it works either way).

The choke acts to prevent or reduce RF on the outside of the coax headed back to the shack.

Anytime between the choke and the entry point to the house/shack is where you need your lightening protection (polyphasor).

In the case of your variant of the G5RV (I use an MFJ version, no fixed length of coax is specified) if the 70' is required as part of the antenna design or match, then roll up your 10 turns out of that 70', and apply the polyphasor between the 70' and your shack. Rolling it up to make the choke shouldn't harm the effect of your 70' section of coax.

Yup - I don't think you want that lightening surge suppressor ANYWHERE INSIDE your house.

Mike Y
KM5Z
Dallas, Texas
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N1LO
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« Reply #3 on: June 23, 2008, 10:53:21 AM »

Walt,

I would: save the money on the lightning arrestor, connect the antenna to the rig only when actually operating, connect it to ground when not, and lastly, of course, not operate whenever I could hear thunder.

You'll need the extra dough for good grounding materials!

Mark
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KB9CRY
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« Reply #4 on: June 23, 2008, 12:45:24 PM »

I assume this should go on before it enters the shack?

Yes

So should I spool up 50 feet of the coax

Yes

or should I just put the polyphaser on inside the shack and attach it to a grounded cold water pipe?

NO....it needs to be outside and connected to your SPG. Do a search on grounding on the main eham search forum page.


means I need to wind it up or something.


Yep, you got it.



You need to install four of the five basic elements of a properly designed grounding system:

1) SPG = Single Point Ground (one or more ground rods installed just where cables go into the shack)
2) Lightning Arrestors Installed at the SPG
3) Shack Ground All shack equipment grounded to a buss inside the shack and that then bonded to the SPG.
4) Electrical Service Ground Bond your electrical service ground rod to your SPG.
5) Antenna Mast/Tower Ground Not needed in this case but your tower/mast ground system is also bonded to the SPG.



And this coming from a tower expert makes no sense:

I would: save the money on the lightning arrestor, connect the antenna to the rig only when actually operating, connect it to ground when not, and lastly, of course, not operate whenever I could hear thunder.
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WB6BYU
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« Reply #5 on: June 23, 2008, 01:10:02 PM »

The only two good reasons for the recommended length of
feedline would be:

(1) to increase the losses so the SWR looks better at the
rig, or

(2) to provide an impedance that is easier for common
tuners to match efficiently on one or more bands.

With a standard G5RV on 80m the input impedance is about
12 ohms at the end of the twinlead.  A quarter wave of
50 ohm coax would transform this to 200 ohms:  the SWR
doesn't change (except due to loss) but tuner efficiency
is likely to be better.  And many of the older tube rigs
could match a load of 200 ohms but not 12 ohms.  Hence the
recommended line length.

But other than that there is no magic to it, and in the
second case it really is the amount of coax between the
twinlead and the tuner that is important:  splicing in
some extra length between the lightning suppressor and
the tuner defeats part of the purpose.

The best approach is to run the existing cable to your
ground rod, then cut the cable at that point and install
PL259 plugs on both ends to mate with the suppressor.  
(Make sure you have allowed enough length for winding a
choke balun at the feedpoint.)  Then use the remaining
length to go from there to the tuner.  If you have
extra, it doesn't really matter which piece is too long.

This gives you the desired length of coax from the twinlead
to the antenna tuner with the suppressor in the middle
of it.

One other suggestion:  check the SWR and/or tuner settings
with the suppressor isolated from ground, then again with
it properly grounded.  If the SWR changes for the same
tuner settings, you have RF on the coax and you may want
to improve your choke balun.

Otherwise if your tuner can match the impedance with
reasonable efficiency, there is no reason to use a
specific coax length with this antenna.
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K1CJS
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Posts: 6034




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« Reply #6 on: June 23, 2008, 01:29:51 PM »

"Anytime between the choke and the entry point to the house/shack is where you need your lightening protection (polyphasor)."

I would have to disagree with that--the proper point to mount lightning protection is outside the house/shack, just before or at the point the co-ax enters the building.  Yes, I know there are differences of opinion on that, but I believe the polyphasor company recommends what I just related.
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WALTERB
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Posts: 528




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« Reply #7 on: June 23, 2008, 01:44:56 PM »

My main problem is the coax will (or plan to) enter the house/shack almost dead center of the house. The roof of the house and roof of the garage are at different heights and that is where the coax enters the house (or I should say where I had planned on having it enter.)

I could run coated #4 copper line along the roof of the house down to a grounding post? But that doesn't really give the lightning a direct path to the ground.

I could also run the coax to the end of the house to a grounding rod and polyphaser, then have add 50 more feet of coax to run in back thru the roof of the garage and into the basement for a total of 120 feet of coax?

thanks
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KB9CRY
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« Reply #8 on: June 23, 2008, 03:20:44 PM »

 The roof of the house and roof of the garage are at different heights and that is where the coax enters the house (or I should say where I had planned on having it enter.)

No, bad bad.  The coax should drop to ground to your SPG and your arrestors and then be routed into the house to the shack.  NEVER EVER (emphasis on purpose) enter directly into the house at any elevation.



I could run coated #4 copper line along the roof of the house down to a grounding post? But that doesn't really give the lightning a direct path to the ground.

Correct.  The coax needs to be dropped to the ground first.


I could also run the coax to the end of the house to a grounding rod and polyphaser, then have add 50 more feet of coax to run in back thru the roof of the garage and into the basement for a total of 120 feet of coax?

That would work also.  For the lengths given, you'd never see/hear a difference.
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K1BXI
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Posts: 812




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« Reply #9 on: June 23, 2008, 03:21:39 PM »

Lots of G5RV experts here....so tell us why that 30 ft section of twin lead radiates and why it only shows 12 ohms impedance om 80 meters at the coax end. Don't just guess, give us some good transmission line theory.

John.....K1BXI  
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WALTERB
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Posts: 528




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« Reply #10 on: June 23, 2008, 03:39:34 PM »

Ok,  I’m pounding this out in my head. So far so good.  I have a ranch house with a 2 car garage attached to the end of the house so its one long rectangle.  The new plan (if it survives this forum thread) Is to  set up two Channel Master 30 foot telescoping masts basically at each end of the house. I can run the G5RV between them basically over the house or off slightly at an angle in a North South direction.  The feed point is in the middle but with lightning protection in mind I will run the 30 feet of ladder line  and 70 feet of coax towards the garage end of the house and the mast.  Since the antenna is 102 feet long, I will run out of ladder line long before I reach the mast. The feed line turns to coax after 30 feet and I will run to the mast while loosing elevation along the way from the feed point.   Then I’ll run it down the mast to the ground, where I will have a polyphaser lightning arrestor attached to a 5/8 copper clad grounding rod that is pounded into the ground about 8 foot.  

So far so good?  

Next I want to run a new section of coax (I assume 50 ohm to stay consistent)  from the polyphaser and grounding rod up the side of the garage to the eve of the roof where it enters the grarage, then across the inside of the roof of the garage to the house and down into the basement / shack.

How does this sound?

thanks
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K1BXI
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Posts: 812




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« Reply #11 on: June 23, 2008, 04:12:01 PM »

Walt...I hope that's your name, with your antenna completely over the house at no more than 20-30 ft separation you may have some rfi problems both transmit and receive. If there is any way to keep it from being directly over the house, I would do it.

Don't worry about the direction it runs, at 30 ft it will mostly be a high angle radiator anyway. I know this doesn't answer your question about the Polyphaser which has been covered by others here.

The old saying holds true here "Any antenna is better than no antenna" so do the best you can and have fun.

John.....K1BXI
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WALTERB
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Posts: 528




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« Reply #12 on: June 23, 2008, 04:56:48 PM »

I could run it at an angle away from the house. My yard is 90 wide so it will have to be at bit of an angle anyway. the yard is plenty long, but I have to navigate trees.

thanks
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WB6BYU
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Posts: 13238




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« Reply #13 on: June 23, 2008, 05:15:53 PM »

If I understand your proposed layout the feedline will run
back to one of the end supports, roughly parallel to the
antenna wire.  Is this correct?

That isn't generally a good idea.  Ideally the feedline
would run off about at right angles to the antenna for
some distance (the further the better, up to 1/4 wavelength
or so.)  It would be better to drop the feedline down to
the roof at least, and perhaps from there to the ground
roughly at right angles to the direction that the antenna
runs, or at least along the edge of the roof rather than
right under the antenna.  This minimizes the induced RF
currents on the outside of the coax.

It isn't always possible to keep the coax out of the field
of the antenna, but it tends to reduce the chances of
quirky behavior.


While I agree that the twinlead portion shouldn't
radiate when the antenna is properly balanced (except
for a very small amount due to the finite spacing between
the wires), I do stand by my 12 ohm value as a reasonable
estimate of the 80m impedance at the end of the matching
section.  This is based on both an EZNEC simulation that
showed about 12. + j0 ohms at 3.75 MHz, and practical
experience with an antenna where minimum SWR on 80m was
about 4 : 1.  The actual impedance seen at the tuner
end of the coax, of course, will vary with the line length.  
The 12 ohms will vary with the specific set of antenna
dimensions used and the impedance of the twinlead.  YMMV
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KB9CRY
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« Reply #14 on: June 23, 2008, 06:07:55 PM »

Next I want to run a new section of coax (I assume 50 ohm to stay consistent) from the polyphaser and grounding rod up the side of the garage to the eve of the roof where it enters the grarage, then across the inside of the roof of the garage to the house and down into the basement / shack.

How does this sound?


Why not run the coax as described but route it around the outside of the house and then to your SPG and arrestor and then into the basement and the shack.  Technically you want a shortest distance as possible between the SPG and your shack entrance.  The potential issue is if your arrestor is further away, that section of coax between the far away arrestor and your equipment could pick up induced energy from a nearby strike.

Also you haven't mentioned the addition of the other basic elements of a grounding system that I itemized in another post.  If you leave out ANY one of those and all may be for naught.

That is where the naysayers are partially correct.  All of their stories about someone with grounding, etc got zapped, upon investigation, would reveal that some element of the basics was not installed.  It all works together and you really need all elements for it to work.
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