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Author Topic: DX Antenna for 40/80 meters  (Read 8339 times)
N0DSN
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Posts: 13




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« on: February 09, 2011, 04:40:53 AM »

I am considering putting up an antenna for 40/80m DX this year. Station consists of Kenwood TS-2000 and a Mosley TA-33 Jr Warc for 10-20 100 watts. I do have a Buckmaster OCF Dipole up at 30ft I have been using for these two bands. Looking to get something with better performance. Wonder what would be a good way to go? I have been considering a loop but just don't know what would be best. Let me know your thoughts and what works for you!
Thanks
N0DSN
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WB6BYU
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Posts: 13457




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« Reply #1 on: February 09, 2011, 06:13:39 AM »

Unless you can get it up high enough, you probably want vertical polarization (though
performance depends on the local ground conductivity.)

For a simple antenna I've had good luck with a wire vertical that is 1/4 wave on 80m
and 1/2 wave on 40m.  I built a matching network that automatically matches it
on both bands at the same time (using 2 capacitors and one coil).

While a straight vertical radiator is great, this also works with a T shape:  basically
you make it as tall as possible, then use a horizontal wire (or sloping downwards
slightly) as a top hat to bring it to resonance.  An inverted L shape (30' up, 30'
horizontal) will also work but has more high angle response, especially on 40m.

Because you are going to be using a tuner at the base anyway, the exact wire
length isn't critical, but the match on 80m is better if you err slightly on the long
side.  That top loading wire, however, shouldn't be more than half wave long on
40m or the radiation suffers.  Something like a 40' vertical with two 15' top loading
wires looks like a good combination.  The wires can angle downward and be part
of the guy system if needed.
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W7ETA
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Posts: 2527




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« Reply #2 on: February 09, 2011, 02:33:52 PM »

Check out W4RNL's site.

He has a great article about verticals and self contained low band antennas.

Best from Tucson
Bob
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WX7G
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Posts: 6197




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« Reply #3 on: February 09, 2011, 06:13:24 PM »

For 40 meters something as simple as the Hustler 4BTV vertical will give you good performance. On 80 meters though the radiation efficiency is 15% or so. The MFJ-1792 80/40 meter vertical will give better performance on 80 meters. For either antenna two dozen 20' radials is a good start.
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NK6Q
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Posts: 202




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« Reply #4 on: February 09, 2011, 10:04:17 PM »

What about a W3DZZ antenna?  I heard it was a good performer on 80 & 40.  Also, isn't the noise floor inherently higher with verticals?

Bill in Pasadena, NK6Q
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WX7G
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Posts: 6197




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« Reply #5 on: February 10, 2011, 12:57:52 AM »

The OFC Dipole and the W3DZZ are both dipoles and as such will perform the same.

The top loaded 33' MFJ vertical has a modeled radiation efficiency of 60% over a 20 ohm counterpoise. Average ground is used in the model.

Compared to a dipole up 30' the vertical has more gain at (TOA) take-off-angles below 17 degrees. Here is the gain of the vertical relative to the dipole:

10 deg TOA  2.5 dB
 5  deg TOA  5 dB
 
The vertical will produce a better signal at the other end. Will it receive more noise such that the signal-to-noise ratio is worse than the dipole? It might. If so the dipole can be used for receive and the vertical for transmit.     
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N0DSN
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Posts: 13




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« Reply #6 on: February 10, 2011, 03:50:58 AM »

Thanks for the replies. I will have to look at the vertical. I did check out W4RNL's website. Lots of info that I will have to look at. Thanks again.
N0DSN
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WX7G
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Posts: 6197




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« Reply #7 on: February 10, 2011, 06:26:08 AM »

What will get you low band DX is an amp. Replace your antenna and you're looking at 3-6 dB. Get an AL-811 amp and you get 8 dB. Put them together and you have 11-14 dB. The will bring you from below the noise level to sufficient copy to complete a (CW) QSO.

I DX on 80/40 using a 23' base loaded vertical. The efficiency is 60% on 40 meters and 20% on 80 meters. With 500-600 watts behind it I work DX everyday including Europe, Africa, Asia, and VK/ZL on 80 meter CW.

For low band DXing you need three things: Antenna, power, and mode. CW = DX. PSK-31 = DX. SSB = no DX.
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KB4MB
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Posts: 295




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« Reply #8 on: February 10, 2011, 06:39:39 AM »

Of course you can DX on 80 with SSB and 100 watts.  However, you do have to like being disappointed a whole bunch!  If you want consistent DX, cw and an amp is the way to go.

But remember, a simple inverted L or vertical is not hard to put up if you have trees around your house.  I have a coax trapped inverted L for 80 and 160, I work DX on 80 with it (South Africa, Eastern Europe) consistently every night within a few calls - it would certainly work better with power behind it, but I just want to make the suggestion that you don't have to BUY anything, and the time invested is interesting and cheap.  Best time to lay radials is now anyway, you can put them on top of the lawn, make your own lawn staples out of stiff wire and in the next few months they will be gobbled up by the lawn.  The coax trap wasn't made with too much fuss, just some large pvc and I set its' resonance just below and between the bands in question.  Lot of guesstimating on that one, and dirt cheap!
« Last Edit: February 10, 2011, 06:41:31 AM by KB4MB » Logged
N0DSN
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Posts: 13




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« Reply #9 on: February 10, 2011, 06:51:38 AM »

Thanks for the help. Getting good information means alot. As far as an amp goes I would love to get one but just want to finish with antennas before adding power. And I do have the Al-811 in mind to get. As far as cw goes. I currently don't operate cw. In the process of learning now. Looks like I got some work ahead of me. But enjoyable non the less.
N0DSN
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WX7G
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Posts: 6197




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« Reply #10 on: February 10, 2011, 01:14:17 PM »

Homebrew lawn staples are nice but it takes time to make hundreds of them. DX Engineering has plastic biodegradable lawn staples made for radials. They are large and have a wide area to push on with a palm. The look medieval in fact. 100 cost $20. I use one every 4' and intalling 1000' of #14 stranded house wire used 250 staples. Cut the lawn super short and install the radials. Then set the lawn mower to a high setting. In a few months there is no trace of the radials. the staples desolve in a year or two. Good DXing!
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WB6BYU
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Posts: 13457




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« Reply #11 on: February 10, 2011, 01:19:13 PM »

If you are limited to about 30' of vertical height, then a 40m loop may work well, but one
for 80m will be more difficult to arrange.  With a 50' support an 80m loop becomes more
practical.  Generally speaking vertical loops aren't conductive to multiband operation, as
W4RNL points out.
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KB4MB
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Posts: 295




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« Reply #12 on: February 10, 2011, 01:46:04 PM »

Quote
Homebrew lawn staples are nice but it takes time to make hundreds of them.

True... sometimes though cheapness wins out.  I have a roll of galvinized electric fence wire since I have a horse farm.  I just sat with a pair of dykes and cut 50 of them in a period of 10 minutes.   If I was in front of the tv, I would have done more Smiley  Same thing, did a staple every 4 feet.  Good to go.  It is nice there are alternatives though!
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K4RVN
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Posts: 785




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« Reply #13 on: February 10, 2011, 02:48:05 PM »

N0DSN
One of the cheapest and best wire antennas for 40 meter DX is a half square. I have one that I use on 40 meters.
They are about the length of a dipole actually a pair of phased verticals of wire  requiring no radials and should work just fine
at 30 ft if that's all you have. The vertical ends can be bent to provide clearance at ground level. I plan to put up another one for additional gain using a reflector. I bought mine from K4TR who makes them in Fl.
Here is a link which describes the half square. Take a look if interested in phased verticals , no radials, a real DX antenna.
02/11/2011 Worked EA7GAK in Spain with my half square at 400 watts. My report 59 + 5
His 59 +15 . The antenna is a DX antenna.

http://www.dxzone.com/cgi-bin/dir/jump2.cgi?ID=20962


K4RVN
« Last Edit: February 11, 2011, 04:09:59 PM by K4RVN » Logged
N4JTE
Member

Posts: 1158




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« Reply #14 on: February 10, 2011, 03:31:17 PM »

Considering your height is constricted to 30 ft. and you want to work DX on 40 and 80 meters my feeling is that any horizontal antenna might be disappointing, especially on 75. To compensate you need to head towards the vertical approach, thinking 40 for starters, the half square is one alternative but really still low as the vertical drops and take off angle will still be contingent on near and far field ground reflectivity and, as designed, is a 180 degree phase line broadside two element vertical antenna, about 1db over a dipole same height, but it would still be a good build to see any improvement while keeping OCD in the air for comparison.
As far as 80 and DX, with your height restrictions, it will take a well designed inverted L to have a chance for the occasional times when your noise floor will allow to you to hear and work your definition of DX. My suggestion is to leave up the OCD for receiving and if your lucky, any type of vertical configuration will give you a shot on transmit.
Regards,
Bob
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