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Author Topic: 80 meter dipole issues revisted  (Read 1816 times)
KB2DO
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Posts: 53




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« Reply #15 on: April 09, 2010, 02:25:12 PM »

seems to me a 3:1 vswr at the ft2000 is a problem.
have u tried to find out why?
Would this cut back your power?

73,
robbie
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N5XO
Member

Posts: 104




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« Reply #16 on: April 09, 2010, 02:50:30 PM »

The original antenna that I pulled down was a dipole with ladder line, this new one has the twin lead that terminates into the coax connector.

It is the G5RV-CL made by Radiowavz.com

It is 102 ft and supposed to be 10 to 80 meters. My only desire is 80 meters with this, as the rest of my antennas are tri-bander beams or mono-bander beams.

It is 102 feet and uses 300 ohm heavy duty stranded copper.
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N2EY
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Posts: 3895




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« Reply #17 on: April 09, 2010, 04:00:32 PM »

The G5RV is a 20 meter antenna I know people use them for a multi-band antenna but they are desinged for 20.

No, that's just not true.

The G5RV is a multiband antenna system, designed by its namesake for multiband use. I have the original article and the followups; he says so himself.

The problems hams have with the G5RV are all based on a lack of understanding of the compromises it involves. It was designed back when 30, 17 and 12 meters weren't ham bands, and when most ham rigs would match almost anything with less than about 5 to 1 SWR.

The problem is that many hams think that it should provide an SWR of 2:1 or less from bandedge to bandedge on every HF amateur band. And that's not the case.

----

IMHO, the solution to the OP's problem is a plain half-wave coax-fed dipole.


73 de Jim, N2EY
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N5XO
Member

Posts: 104




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« Reply #18 on: April 09, 2010, 05:14:24 PM »

seems to me a 3:1 vswr at the ft2000 is a problem.
have u tried to find out why?
Would this cut back your power?

End of Quote:==========================

That is why I run things through a tuner.
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N5XO
Member

Posts: 104




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« Reply #19 on: April 09, 2010, 05:16:24 PM »

At least we all have the same idea. But I wonder what the coax is (RG- 213 or better hopefully) and would it be possible for him to disconnect the coax at the antenna end and put the dummy load out there and see what the result would be.

End of Quote:=================================

I tried that, I put a full legal limit dummy load out side at the end of the coax run, crank up perfect SWR and the amp cranks out full 1400+ on 80 meters with the dummy load there.

Hook the antenna back up and the issues as described.
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K1BXI
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Posts: 812




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« Reply #20 on: April 09, 2010, 06:51:51 PM »

You said "I put a full legal limit dummy load out side at the end of the coax run, crank up perfect SWR and the amp cranks out full 1400+ on 80 meters with the dummy load there."

I presume when you said you "cranked up a perfect SWR" you had the tuner in line with the dummy load. If that is the case, I believe your problem is with the tuner. Being, for some reason, unable to handle the reactive loads presented to it by either antenna system you have tried. I think it should have tuned out the reactance, but something may be amiss in it.

But........ since you only want this antenna for 80/75 perhaps a better choice, like Jim, N2EY just said, is a simple 1/2 wave center fed dipole with just your coax for the feed line.  If you are short on space you can let a few feet on each end hang down.

I wouldn't make it less than 120 ft long, it should resonate it around 3.900. I think your tuner would tune it fine on the low end also.

If you have room make it 126 ft long and resonate close to 3.750, the middle of the band. The SWR at the band edges may be 5:1, but the loss in your coax at these frequency's is negligible. Even with a 10:1 SWR.

Good luck..........John
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KG4GPJ
Member

Posts: 6




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« Reply #21 on: April 09, 2010, 09:32:18 PM »

Quote: "The current antenna is a G5RV styled antenna 102 ft long.

I'm feeding it with new LMR400 {I consider this minimum cable period} through an MFJ 989D tuner.

With out the tuner it is showing 3.1 SWR at 3.823 on the FT2000D, but on the tuner meter it is showing 1.9."
End Quote

If the rig is showing a relatively high SWR with the amp operating, then perhaps the input coil on the amp for the 80 meter band needs to be adjusted so that the rig will see a lower SWR.  The rig may be folding back power (probably is if the rig is seeing an SWR of >3.0:1 going into the amp.  The tuner's SWR meter is showing the match between the tuner and the feedline, not the match between the rig and the amp.  If the rig's internal SWR meter is showing that high of an SWR with the amp inline and in the "operate" position, it's telling you something.  What's the SWR on the rig's internal SWR meter with the amp in "standby" position and the rig running barefoot?

I have an Ameritron amp and the tuning coils for a couple of bands need some adjustment, which is reflected in a little higher SWR on the rig's SWR meter while the amp is operating than it should be (but nothing like 3.8:1!).

Larry
KG4GPJ
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WB6BYU
Member

Posts: 13353




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« Reply #22 on: April 09, 2010, 09:56:44 PM »

OK, we're making some progress.  Your current antenna is another G5RV, so the
impedance seen by the tuner will be about the same as it was before.  The
impedance at the end of the matching section is on the low side, and 3 : 1 SWR
is probably as good as you can expect (ignoring loss in the coax.)  Modeling this
with 100' of coax gives a relatively tame load to match.

The symptom as I understand it is that the SWR jumps when you get the output
power over 400 watts when using the tuner, but when feeding direct to the dummy
load without the tuner you can run full power.

Does the jump in SWR show on both SWR meters (the one on the amp and the one
in the tuner)?  Or just one of them?

You said you had tried it with your Henry amp - does this have a manually tuned
output?  If so, it should be able to match the impedance of the G5RV in spite of
the fact that the SWR is 3 : 1.  (My model puts the impedance around 95 + j70.)
If you do this, can the amp deliver full power to the antenna?  Try it without the
tuner in the circuit, and with the tuner inline but bypassed.

The next step is to put the dummy load on the output of the tuner and tune it
for minimum SWR (actually using the tuner circuit, not in bypass mode.)  Does the
amp still put out full power, or do you still have the same problem when matching
the dummy load as when matching the G5RV?


Have you tried using a different jumper cable between the amp and the tuner?

If the amp puts out full power into the antenna without the tuner, but not when it
is in line, there are a couple possibilities.  The first is that the jumper cable between
the tuner and the amp is defective.  If you are seeing the high SWR on the meter in
the tuner, that would point to a problem in the tuner,  especially the portion of the
circuit between the SWR meter and the output connectors.  Besides the common
problem of arcing, you could also have a cold solder joint or other poor connection.
(You wouldn't be the first person to find that one of the joints isn't soldered in an
MFJ tuner.)  This could work OK at low power levels, but fail at higher currents.
Dirty or arced contacts in the antenna selector switch or the inductor inside the
tuner could cause similar symptoms.  That's why I suggest using the tuner to match
the dummy load, as it would test this part of the circuit.  (But it might not use the
same value of inductance as the G5RV, in case there is a poor contact at just spot
on the coil.)

Further thoughts:  if you were feeding your G5RV through a 4 : 1 balun before, that
would increase the SWR on the coax even more (because the impedance is already
down around 16 ohms, and the balun would lower it to 4 ohms if it actually worked
at such low impedances.)  A 1 : 1 current balun is a much better choice in this
application.  Since it sounds like you are not using a balun at present, you could have
some common mode RF on the coax (though this would dissipate somewhat if the
cable runs along the ground.)  If the rig is sensitive to RF (which I don't know, but
I haven't seen ruled out) then this would exacerbate the problem.  Winding some
of the coax into a choke balun may help in that case.

But I'd go back to the tests I suggested:  if the Henry can load the G5RV directly
without the tuner in line at all, but runs into problems when you run it through the
tuner in bypass mode, look for problems in the tuner wiring, especially the antenna
selector switch and output connectors.  If it works fine in bypass mode but not
when matching the dummy load, look for a bad connection on on the switch, the
inductor, or the wiring of the individual tuner components.

Other combinations of symptoms might lead to different conclusions, but driving the
G5RV directly without the tuner, and adjusting the tuner to match the dummy load,
should together help to pinpoint the source of the problem.

Good luck!
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N5XO
Member

Posts: 104




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« Reply #23 on: April 10, 2010, 05:02:28 AM »

If the rig is showing a relatively high SWR with the amp operating, then perhaps the input coil on the amp for the 80 meter band needs to be adjusted so that the rig will see a lower SWR. The rig may be folding back power (probably is if the rig is seeing an SWR of >3.0:1 going into the amp. The tuner's SWR meter is showing the match between the tuner and the feedline, not the match between the rig and the amp. If the rig's internal SWR meter is showing that high of an SWR with the amp inline and in the "operate" position, it's telling you something. What's the SWR on the rig's internal SWR meter with the amp in "standby" position and the rig running barefoot?

I have an Ameritron amp and the tuning coils for a couple of bands need some adjustment, which is reflected in a little higher SWR on the rig's SWR meter while the amp is operating than it should be (but nothing like 3.8:1!).

End of quote:------------------------------------------

Sorry for if I was not clear, no the radio has no SWR issue, the tuner will tune up with 200 watts from the radio just fine, but when I crank it up the amp and load up the amp as I cross over say 400 watts, the SWR's suddenly shoot up on the tuner meter.

and I'm unable to get more than 400 out of the amp with the antenna as the load.

I've got a Drake tuner, I'm going to swap around for the MFJ and see what happens....
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N2EY
Member

Posts: 3895




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« Reply #24 on: April 10, 2010, 06:40:03 AM »

the tuner will tune up with 200 watts from the radio just fine, but when I crank it up the amp and load up the amp as I cross over say 400 watts, the SWR's suddenly shoot up on the tuner meter.

and I'm unable to get more than 400 out of the amp with the antenna as the load.

I've got a Drake tuner, I'm going to swap around for the MFJ and see what happens....

I predict that the MFJ tuner is the problem. Something inside it is saturating, arcing, or changing characteristics when serious power is applied.

If the Drake tuner is an MN-2000, you should have no problems.

But as I previously wrote, if the antenna is only for 75/80 meters, you're much better off with a simple coax-fed dipole or inverted V, fed directly with RG-213 or similar coax. Such an antenna will give you 2:1 SWR over about 200 kHz of the band, and with a simple matching network will cover the whole band. There are also ways to make a simple coax-fed dipole have low SWR over the whole band but I won't go into that here. Yet.

The G5RV is meant for applications where a ham has limited space and needs one antenna to cover several HF bands. It doesn't sound like you need that.

73 de Jim, N2EY
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W4VR
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Posts: 1194


WWW

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« Reply #25 on: April 10, 2010, 06:42:04 AM »

I had the same problem recently using a 40 meter dipole.  It turned out to be a bad balun and also a bad PL-259 connector that plugged into my coax switch.  It took me a while to figure out the bad connector because when I unsoldered it I could not see where it was bad.  I simply replaced it with a new one and everything is now working fine.  Ron
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KB2DO
Member

Posts: 53




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« Reply #26 on: April 10, 2010, 07:11:40 AM »

I am sorry but need u to verify low vswr between the
transceiver and end of jumper that goes to the amp.
put the dummy load at the end of that jumper and see
what the reading is.
You said the reading was 3:1 at the ft2000 meter.
I was thinking u have a bad jumper and it takes little time to check.

If that is ok then move on to a simple dipole cut for 80
in the range u are going to operate.
No tuner needed, at resonance ur amp will tune up ( or should ).
U can add the tuner later if all is well.

73 and happy hamming,
robbie
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