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Author Topic: 40m Vertical as a 6m Antenna?  (Read 815 times)
EI4HQ
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« on: March 14, 2008, 11:45:12 PM »

Hi,

I only recently got a rig that has 6 metres on it, and I currently don't have an antenna for that band.

I was messing around in the shack the other day and for the heck of it I checked to see if any of my existing antennas (individual ground mounted verticals for 160, 80 & 40) would tune up on 6m. Interestingly, the 40m GP tunes very nicely - without an ATU in-line the SWR is 1.3 at 50.0MHz and 3.1 at 51.1MHz. Very useable on CW, and across the band with an ATU in-line.

the 40m GP is approximately 2 wavelengths long on 6m.

My questions are these:
1. What will the radiation pattern look like?
2. Does it have some gain?

Unless I've missed something, the ARRL Antenna Book doesn't discuss the behaviour of ground mounted vertical antennas when they are multiple wavelengths long... are they behaving as 'long-wires' in this situation?

Rgds
Cormac, EI4HQ
IO51UU
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N3OX
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« Reply #1 on: March 15, 2008, 09:33:08 AM »

"are they behaving as 'long-wires' in this situation? "

Yes, they are, but the longer the wire the more they tend to fire *along* the wire instead of broadside.

In the case of a 40m groundplane, it wants to fire *UP*  Since it's only a couple of wavelengths long, it actually just fires strongly a bit above 50 degrees elevation.

Check it out: http://www.n3ox.net/files/40GP_on_6m.jpg

The blue curve (sorry, I know the color contrast isn't great, it's the curve that the green dot is on) you can see it has some residual low angle lobes but they're basically worse than if you ground mounted a 6m vertical, which would be a really terrible antenna.

The black curve shows a simple 6m dipole made of 25mm aluminum tubing mounted at 20 feet above ground ... you can see that at useful angles, it just blows the 40m antenna away.

Your 40m GP has plenty of gain if you want to work 6m satellites, but generally, it's a difference of night and day working 6m with a random HF antenna vs. a "real" 6m antenna, even as simple as a dipole on a TV-type rotator on your house.

73,
Dan





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73,
Dan
http://www.n3ox.net

Monkey/silicon cyborg, beeping at rocks since 1995.
AA4PB
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« Reply #2 on: March 15, 2008, 10:09:38 AM »

To make matters worse, the vertical polarization will be wrong for working SSB and CW as those stations generally use horizontal polarization. Being cross polarized will result in many dB of attenuation in most cases.

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WB2WIK
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« Reply #3 on: March 15, 2008, 01:26:35 PM »

Indeed, a 40m vertical is a lousy 6m antenna.

"Loading up" well and providing a good match (low VSWR) is no indication of performance.  A 1/4-wave six meter ground plane mounted on your roof will work many times better than the 40m ground-mounted vertical.

As Bob pointed out, everybody on 6m using SSB and CW (or really any weak signal mode including WSJT modes) are using horizontal polarization, and the cross-polarization attenuation from being vertically polarized is terrible.  Doesn't matter for "skywave," but then 90%+ of all six meter contacts are not skywave, they're tropo and for tropo paths polarization holds for hundreds of miles.

A simple rotary 1/2-wave dipole can be built for $10 (plus a support and a rotator) and will work far better for 6m SSB/CW than any sort of vertical will.

WB2WIK/6
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EI4HQ
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« Reply #4 on: March 15, 2008, 02:03:10 PM »

Thanks to one & all for info. provided.

I had a feeling there was a fatal catch - like was pointed out, just because it loads don't mean it'll be any good.

After I posted, it dawned on me that I had a copy of EZNEC somewhere. I dug it out & I modelled the system. As was pointed out the main lobe is somewhere in the 50/60 degree range. Coupled to the polarisation issue, its basically hopeless, as you've all advised.

Case closed. I was never considering it as a serious candidate for a 6m antenna, but I'm an inverterate tinkerer & had to know. I'll build a decent beam one of these days.

Rgds
Cormac, EI4HQ
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WW5AA
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« Reply #5 on: March 16, 2008, 04:31:46 AM »

Unless you get really involved in working 6 meters, the GP will be just fine when the band is open. A wet noodle works with a big opening. I have 61 confirmed on 6 meters using an all band doublet just because my last two radios had 6 meters. Have fun.

73 de Lindy
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N3OX
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« Reply #6 on: March 16, 2008, 06:05:39 PM »

"Unless you get really involved in working 6 meters, the GP will be just fine when the band is open. A wet noodle works with a big opening."

Boy, think how boring 6m would be if every 6m op had that philosophy ;-)

A decent 6m antenna can be built with $30 of hardware store parts.  Why not put up a decent 6m antenna to go with your rig's capability?

http://www.n3ox.net/projects/sixmoxon

Comparisons between that and my 100 foot magnet wire doublet with a tuner right at the feedpoint when I was in an apartment favored the Moxon by maybe 10, 15dB.

Dan
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73,
Dan
http://www.n3ox.net

Monkey/silicon cyborg, beeping at rocks since 1995.
WW5AA
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Posts: 2086




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« Reply #7 on: March 17, 2008, 07:37:20 AM »

"Boy, think how boring 6m would be if every 6m op had that philosophy ;-)"

As said Dan, I am not a 6 meter op and wouldn't go out of my way to set up a good 6 meter antenna. However Charlie Ho (VR2XM) didn't think I was boring a couple years back, just a little weak (:-)

73 de Lindy
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N3OX
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« Reply #8 on: March 17, 2008, 10:51:39 AM »

"As said Dan, I am not a 6 meter op and wouldn't go out of my way to set up a good 6 meter antenna"

Yeah, Lindy, I didn't really mean that comment to be directed to you or anyone on this thread really.  I know you know the value of a good antenna and would put one up if you wanted to.  I know Steve does and clearly Cormac does as well...

It's more for the sake of the general readership, because it sometimes seems like people don't appreciate the overall link budget in ham radio ;-)

The DX QSO's I'm proudest of are those that are made with stations equipped about the same as me.  It means that the two of us are sharing the station burden, and using our skills and decent stations to bridge the gap.

There's nothing wrong with using marginal antennas, low power, etc if:

1) That's what you can get up.  I operated from an apartment, I'd much rather be on the air with an antenna that requires the DX to do the heavy lifting than not be on the air at all.

2) You don't worry much about those bands.  I never quite understood this one just because I get a little itchy if my radios have a band I don't have a decent antenna for.  I was without a real 12m antenna for a long time since I moved to the current QTH and it really bothered me ;-)  Doesn't bug everyone though...

or

3) You're doing it just for fun.  Working ZL on 30CW on my magloop was a really fun QSO.  Working LA7THA using 5W on 160m was also fun.  Contacts that take advantage of good propagation and the other station's ears can be great (especially for the other end... I know I'd get excited if I heard a 5W DX station on 160m!)

- - - - - -

The thing that I think is a shame is the guy who puts up a G5RV at 20 feet on his hundred acre farm, never tries anything else, and uses it on all bands just because it makes contacts there.

I know that's not present company's style... but stations with big, or at least decent antennas make ham radio DXing lots of fun for people who really have severe restrictions, so I'm going to keep nudging people to put up "real" antennas ;-)

73,
Dan
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73,
Dan
http://www.n3ox.net

Monkey/silicon cyborg, beeping at rocks since 1995.
WB2WIK
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« Reply #9 on: March 17, 2008, 01:34:47 PM »

>RE: 40m Vertical as a 6m Antenna?  Reply  
by WW5AA on March 17, 2008  Mail this to a friend!  
"Boy, think how boring 6m would be if every 6m op had that philosophy ;-)"

As said Dan, I am not a 6 meter op and wouldn't go out of my way to set up a good 6 meter antenna. However Charlie Ho (VR2XM) didn't think I was boring a couple years back, just a little weak (:-)

73 de Lindy<

::See, but if you had a better antenna you would have copied Charlie's call correctly, which is VR2XMT.  I'm guessing the "couple of years ago" was probably really November 2001, when "Hong Kong Charlie" was pinning my S-meter on 50.115 here in L.A.  I worked him, too -- easily.  Those kind of conditions occur about a few times every 22 or 33 years.

WB2WIK/6

 
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