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Single Band 40 Meter Dipole

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Phil Hystad-3

Single Band 40 Meter Dipole

OK, lots of discussion on open wire feeders and other ideas so I thought I would throw out a question to soak up some of the Elecraft community knowledge and wisdom.

Given a 40-meter dipole antenna, up about 50 feet, what is the best feed configuration for single band usage to minimize loss and minimize common mode currents and radiation from the feed line?

I am thinking of building such an antenna and I would have no plans for using it on any other band but I would like to use it on the full spectrum of 40 meters with primary focus on the low end for CW.

My current idea is to cut the antenna for 7.1 MHz but I am not sure about the best plan to feed it. I am thinking of 450 ohm window line (aka ladder line) to a 1:1 balun and the coax the rest of the way into the shack. Or, how about 300 ohm twin lead into a 6:1 balun with coax the rest of the way into the shack? Would this be a significant difference? Or, some other combination?

Or, maybe a 1:1 balun right at the antenna feed point with coax the remaining distance to the shack?

Any comments or suggestions?

I am thinking of also raising up two other dipoles. I have room to put up 80, 40, and 30 as separate dipole antennas -- it is actually a little bit more awkward to do a fan dipole so I am leaning on not doing that.

73, phil, K7PEH

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Phil Hystad-3

Re: Single Band 40 Meter Dipole

OK, Is the current mode choke a product or a homebrew thing by K9YC?

On Dec 31, 2011, at 9:32 PM, Jim Miller wrote:

> Coax to the feed point and a CM choke per K9YC right there as well. RG8x would be my choice.
>
> 73
>
> Jim ab3cv

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Phil Hystad-3

Re: Single Band 40 Meter Dipole

Or, is that a common mode choke you mean?

On Dec 31, 2011, at 9:35 PM, Phil Hystad wrote:

> OK, Is the current mode choke a product or a homebrew thing by K9YC?
>
>
> On Dec 31, 2011, at 9:32 PM, Jim Miller wrote:
>
>> Coax to the feed point and a CM choke per K9YC right there as well. RG8x would be my choice.
>>
>> 73
>>
>> Jim ab3cv
>
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Don Wilhelm-4

Re: Single Band 40 Meter Dipole

In reply to this post by Phil Hystad-3

Phil,

What do you want to accomplish (other than just an antenna for 40
meters)? Lowest loss? Broadband? Easy- up? Etc? Those parameters will
determine your best choice.

Easiest IMHO is to just feed it with coax with a current mode choke at
the feedpoint. That should present a less than 2:1 SWR over the entire
40 meter band.

If you have an extremely long feedline (multiples of a half wave), then
you might consider using balanced line cut to some multiple of a half
wavelength (electrical length) and the rest of the way to the shack with
coax - put the common mode choke at the transition between the balanced
line and the coax.
This latter suggestion takes advantage of the fact that an electrical
half wave transmission line has the same impedance at both ends - it
does not matter what the characteristic impedance of the line may be.

But unless you have a very long distance to the antenna feedpoint, the
loss of practical coax runs is rather low on 40 meters, so the easiest
is just to use coax.

73,
Don W3FPR

On 1/1/2012 12:14 AM, Phil Hystad wrote:

> OK, lots of discussion on open wire feeders and other ideas so I thought I would throw out a question to soak up some of the Elecraft community knowledge and wisdom.
>
> Given a 40-meter dipole antenna, up about 50 feet, what is the best feed configuration for single band usage to minimize loss and minimize common mode currents and radiation from the feed line?
>
> I am thinking of building such an antenna and I would have no plans for using it on any other band but I would like to use it on the full spectrum of 40 meters with primary focus on the low end for CW.
>
> My current idea is to cut the antenna for 7.1 MHz but I am not sure about the best plan to feed it. I am thinking of 450 ohm window line (aka ladder line) to a 1:1 balun and the coax the rest of the way into the shack. Or, how about 300 ohm twin lead into a 6:1 balun with coax the rest of the way into the shack? Would this be a significant difference? Or, some other combination?
>
> Or, maybe a 1:1 balun right at the antenna feed point with coax the remaining distance to the shack?
>
> Any comments or suggestions?
>
> I am thinking of also raising up two other dipoles. I have room to put up 80, 40, and 30 as separate dipole antennas -- it is actually a little bit more awkward to do a fan dipole so I am leaning on not doing that.
>
> 73, phil, K7PEH
>

Phil Hystad-3

Re: Single Band 40 Meter Dipole

Don,

goals:

1. Lowest loss.
2. Full 40-meter band coverage but resonant at 7.1 MHz.
3. Assume using a tuner (like K3 or KAT500).
4. Choke common mode currents.

Total feed line length will be on the order of 100 feet for this particular dipole. For the 30 meter dipole I also plan to put up, total feed line length is about 75 feet. My 80 meter dipole will be about 100 feet of feedline.

Before I posted this question to this forum, I would have gone ahead and used the same configuration I used previously when I had my 80 meter doublet up. That antenna was fed by 450 ohm ladder line to a 1:1 balun and then RG-8X to the shack.

But one motivation for my question is weighing the difference in loss of tuner + 450 ohm ladder line and balun versus a direct run of RG8X right to the feed point with a choke balun at that location.
The RG-8X loss is about 2 dB on that 100 foot run. I could switch to LMR-400 (at 0.7 dB loss) but I am curious about other solutions.

73, phil, K7PEH

On Jan 1, 2012, at 5:39 AM, Don Wilhelm wrote:

> Phil,
>
> What do you want to accomplish (other than just an antenna for 40 meters)? Lowest loss? Broadband? Easy- up? Etc? Those parameters will determine your best choice.
>
> Easiest IMHO is to just feed it with coax with a current mode choke at the feedpoint. That should present a less than 2:1 SWR over the entire 40 meter band.
>
> If you have an extremely long feedline (multiples of a half wave), then you might consider using balanced line cut to some multiple of a half wavelength (electrical length) and the rest of the way to the shack with coax - put the common mode choke at the transition between the balanced line and the coax.
> This latter suggestion takes advantage of the fact that an electrical half wave transmission line has the same impedance at both ends - it does not matter what the characteristic impedance of the line may be.
>
> But unless you have a very long distance to the antenna feedpoint, the loss of practical coax runs is rather low on 40 meters, so the easiest is just to use coax.
>
> 73,
> Don W3FPR
>
> On 1/1/2012 12:14 AM, Phil Hystad wrote:
>> OK, lots of discussion on open wire feeders and other ideas so I thought I would throw out a question to soak up some of the Elecraft community knowledge and wisdom.
>>
>> Given a 40-meter dipole antenna, up about 50 feet, what is the best feed configuration for single band usage to minimize loss and minimize common mode currents and radiation from the feed line?
>>
>> I am thinking of building such an antenna and I would have no plans for using it on any other band but I would like to use it on the full spectrum of 40 meters with primary focus on the low end for CW.
>>
>> My current idea is to cut the antenna for 7.1 MHz but I am not sure about the best plan to feed it. I am thinking of 450 ohm window line (aka ladder line) to a 1:1 balun and the coax the rest of the way into the shack. Or, how about 300 ohm twin lead into a 6:1 balun with coax the rest of the way into the shack? Would this be a significant difference? Or, some other combination?
>>
>> Or, maybe a 1:1 balun right at the antenna feed point with coax the remaining distance to the shack?
>>
>> Any comments or suggestions?
>>
>> I am thinking of also raising up two other dipoles. I have room to put up 80, 40, and 30 as separate dipole antennas -- it is actually a little bit more awkward to do a fan dipole so I am leaning on not doing that.
>>
>> 73, phil, K7PEH
>>

Jim Brown-10

Re: Single Band 40 Meter Dipole

In reply to this post by Phil Hystad-3

On 12/31/2011 9:14 PM, Phil Hystad wrote:
> Or, maybe a 1:1 balun right at the antenna feed point with coax the remaining distance to the shack?

No question -- 50 ohm coax wound around one or more #31 or #43 ferrite
cores to form a common mode choke. See
http://audiosystemsgroup.com/RFI-Ham.pdf The choke will completely
kill any radiation or pickup from the feedline. If the antenna were a
lot higher, I'd use 75 ohm coax. And yes, tweak the length for 7.1 MHz.

Also see two applications notes about "antennas for limited space" on my
website. A single 2-wire fan dipole cut for 80M and 40M and fed with 50
ohm coax will also work great on 30M, 17M, 15M, and 12M, and using the
same choke to kill feedline current. To minimize feedline loss on 30,
17, 15, and 12, use RG8 rather than RG8X or RG58. This antenna will work
as well as the single-band 40M dipole on 40M. You WILL need an antenna
tuner, like the KAT3.

http://audiosystemsgroup.com/publish.htm

73, Jim K9YC
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Re: Single Band 40 Meter Dipole

In reply to this post by Phil Hystad-3

Phil, I would put the balun at the center of the antenna and use one that doubles as a center insulator and attach point for the raising rope. The loss for RG-8X will be about 1 db, so you will not notice. I have had very good luck with attaching an 80 meter dipole to the same coax and balun. I have not tried a 30 meter, but I would expect it to work well also. I have tried a 80-40-20 fan with no good results. I find it best to spread the 80 and 40 at least 30 degrees and probably 90 degrees would be better. You could use a second 75/80 dipole for either the phone or CW band or I have had good results by putting insulators in the 80/75 and using alligator clip leads when I want the longer antenna. The antenna coupler should not be needed for 40 meters or for 80/75 for most of the band is you use the jumpers.

Willis 'Cookie' Cooke
K5EWJ & Trustee N5BPS, USS Cavalla, USS Stewart

________________________________
From: Phil Hystad <[hidden email]>
To: [hidden email]
Cc: Elecraft List <[hidden email]>
Sent: Sunday, January 1, 2012 11:13 AM
Subject: Re: [Elecraft] Single Band 40 Meter Dipole

Don,

goals:

1. Lowest loss.
2. Full 40-meter band coverage but resonant at 7.1 MHz.
3. Assume using a tuner (like K3 or KAT500).
4. Choke common mode currents.

Total feed line length will be on the order of 100 feet for this particular dipole. For the 30 meter dipole I also plan to put up, total feed line length is about 75 feet. My 80 meter dipole will be about 100 feet of feedline.

Before I posted this question to this forum, I would have gone ahead and used the same configuration I used previously when I had my 80 meter doublet up. That antenna was fed by 450 ohm ladder line to a 1:1 balun and then RG-8X to the shack.

But one motivation for my question is weighing the difference in loss of tuner + 450 ohm ladder line and balun versus a direct run of RG8X right to the feed point with a choke balun at that location.
The RG-8X loss is about 2 dB on that 100 foot run. I could switch to LMR-400 (at 0.7 dB loss) but I am curious about other solutions.

73, phil, K7PEH

On Jan 1, 2012, at 5:39 AM, Don Wilhelm wrote:

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Jim Brown-10

Re: Single Band 40 Meter Dipole

In reply to this post by Phil Hystad-3

On 12/31/2011 9:36 PM, Phil Hystad wrote:
> Or, is that a common mode choke you mean?

I HATE use of the word "balun," because it is used to describe at least
six very different things. A so-called "current balun"" is really
nothing more than a common mode choke. The chokes I've described in my
tutorial and cookbook are simply much better (and cheaper) common mode
chokes than you can buy for a lot more money.

As Don says, study the tutorial.

73, Jim K9YC
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Mike WA8BXN

Re: Single Band 40 Meter Dipole

In reply to this post by Phil Hystad-3

A table I looked at suggested that on 7 MHz loss for RG8X would be less than
1 db for a 100 foot run of it. When dealing with a resonant dipole on the
lower HF bands feedline loss is not a big issue. I would not consider using
open wire feed for such an antenna (unless I wanted to operate on
non-resonant bands with that same antenna). I'm not sure I would even bother
with any sort of choke or balun. I would not expect RF in the shack or noise
problems. If those issues did appear, then I might look into chokes/baluns.

73/72 - Mike WA8BXN
Happy New Year all!
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Phil Hystad-3

Re: Single Band 40 Meter Dipole

In reply to this post by Phil Hystad-3

Guys,

Thanks for all the responses both on-line here in the group and off-line. Several people pointed me to Jim Brown's (K9YC) paper, including just a few minutes ago, Jim Brown himself (thanks).

I grabbed the paper last night but was too tired from a half-bottle of champagne to do anything but briefly scan the parts about the common mode chokes. Also, I found a power-point talk by K9YC that I will also go through this morning.

So, I am pretty sure I will end up doing the Jim Brown solution. That would be an RG8X feed using one of his common mode toroid chokes (though, I do need to read the paper in detail). I guess finding a source for the ferrite toroids is next on my list although I think I saw one referenced in one of the responses (digikey).

I suppose best placement for the choke is at the center feed point of the dipole but I am a little concerned about the weight. Maybe this is not an issue. By the way, I make my own center insulators with nice screw on brackets and SO-239 connectors. The plastic I use is very tough and I buy it from Tap Plastics.

PEH

On Jan 1, 2012, at 9:13 AM, Phil Hystad wrote:

> Don,
>
> goals:
>
> 1. Lowest loss.
> 2. Full 40-meter band coverage but resonant at 7.1 MHz.
> 3. Assume using a tuner (like K3 or KAT500).
> 4. Choke common mode currents.
>
>
> Total feed line length will be on the order of 100 feet for this particular dipole. For the 30 meter dipole I also plan to put up, total feed line length is about 75 feet. My 80 meter dipole will be about 100 feet of feedline.
>
> Before I posted this question to this forum, I would have gone ahead and used the same configuration I used previously when I had my 80 meter doublet up. That antenna was fed by 450 ohm ladder line to a 1:1 balun and then RG-8X to the shack.
>
> But one motivation for my question is weighing the difference in loss of tuner + 450 ohm ladder line and balun versus a direct run of RG8X right to the feed point with a choke balun at that location.
> The RG-8X loss is about 2 dB on that 100 foot run. I could switch to LMR-400 (at 0.7 dB loss) but I am curious about other solutions.
>
> 73, phil, K7PEH
>
>
> On Jan 1, 2012, at 5:39 AM, Don Wilhelm wrote:
>
>> Phil,
>>
>> What do you want to accomplish (other than just an antenna for 40 meters)? Lowest loss? Broadband? Easy- up? Etc? Those parameters will determine your best choice.
>>
>> Easiest IMHO is to just feed it with coax with a current mode choke at the feedpoint. That should present a less than 2:1 SWR over the entire 40 meter band.
>>
>> If you have an extremely long feedline (multiples of a half wave), then you might consider using balanced line cut to some multiple of a half wavelength (electrical length) and the rest of the way to the shack with coax - put the common mode choke at the transition between the balanced line and the coax.
>> This latter suggestion takes advantage of the fact that an electrical half wave transmission line has the same impedance at both ends - it does not matter what the characteristic impedance of the line may be.
>>
>> But unless you have a very long distance to the antenna feedpoint, the loss of practical coax runs is rather low on 40 meters, so the easiest is just to use coax.
>>
>> 73,
>> Don W3FPR
>>
>> On 1/1/2012 12:14 AM, Phil Hystad wrote:
>>> OK, lots of discussion on open wire feeders and other ideas so I thought I would throw out a question to soak up some of the Elecraft community knowledge and wisdom.
>>>
>>> Given a 40-meter dipole antenna, up about 50 feet, what is the best feed configuration for single band usage to minimize loss and minimize common mode currents and radiation from the feed line?
>>>
>>> I am thinking of building such an antenna and I would have no plans for using it on any other band but I would like to use it on the full spectrum of 40 meters with primary focus on the low end for CW.
>>>
>>> My current idea is to cut the antenna for 7.1 MHz but I am not sure about the best plan to feed it. I am thinking of 450 ohm window line (aka ladder line) to a 1:1 balun and the coax the rest of the way into the shack. Or, how about 300 ohm twin lead into a 6:1 balun with coax the rest of the way into the shack? Would this be a significant difference? Or, some other combination?
>>>
>>> Or, maybe a 1:1 balun right at the antenna feed point with coax the remaining distance to the shack?
>>>
>>> Any comments or suggestions?
>>>
>>> I am thinking of also raising up two other dipoles. I have room to put up 80, 40, and 30 as separate dipole antennas -- it is actually a little bit more awkward to do a fan dipole so I am leaning on not doing that.
>>>
>>> 73, phil, K7PEH
>>>
>
> ______________________________________________________________
> Elecraft mailing list
> Home: http://mailman.qth.net/mailman/listinfo/elecraft
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> Post: mailto:[hidden email]
>
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Don Wilhelm-4

Re: Single Band 40 Meter Dipole

In reply to this post by Phil Hystad-3

Phil,

100 feet of coax (RG-8X) has less than 1 dB loss at 40 meters - RG-213
is less loss, so I recommend using coax unless you have the patience for
properly supporting a balanced feedline. What I am saying is that you
will not gain very much with lower loss balanced line. Be aware when
using the popular window line that it gets lossy when it is wet, in fact
some have reported more loss than small coax.

You should have less than 2:1 SWR over the entire 40 meter band if you
pick the center frequency correctly.

73,
Don W3FPR

On 1/1/2012 12:13 PM, Phil Hystad wrote:

> Don,
>
> goals:
>
> 1. Lowest loss.
> 2. Full 40-meter band coverage but resonant at 7.1 MHz.
> 3. Assume using a tuner (like K3 or KAT500).
> 4. Choke common mode currents.
>
>
> Total feed line length will be on the order of 100 feet for this particular dipole. For the 30 meter dipole I also plan to put up, total feed line length is about 75 feet. My 80 meter dipole will be about 100 feet of feedline.
>
> Before I posted this question to this forum, I would have gone ahead and used the same configuration I used previously when I had my 80 meter doublet up. That antenna was fed by 450 ohm ladder line to a 1:1 balun and then RG-8X to the shack.
>
> But one motivation for my question is weighing the difference in loss of tuner + 450 ohm ladder line and balun versus a direct run of RG8X right to the feed point with a choke balun at that location.
> The RG-8X loss is about 2 dB on that 100 foot run. I could switch to LMR-400 (at 0.7 dB loss) but I am curious about other solutions.
>
> 73, phil, K7PEH
>
>
> On Jan 1, 2012, at 5:39 AM, Don Wilhelm wrote:
>
>> Phil,
>>
>> What do you want to accomplish (other than just an antenna for 40 meters)? Lowest loss? Broadband? Easy- up? Etc? Those parameters will determine your best choice.
>>
>> Easiest IMHO is to just feed it with coax with a current mode choke at the feedpoint. That should present a less than 2:1 SWR over the entire 40 meter band.
>>
>> If you have an extremely long feedline (multiples of a half wave), then you might consider using balanced line cut to some multiple of a half wavelength (electrical length) and the rest of the way to the shack with coax - put the common mode choke at the transition between the balanced line and the coax.
>> This latter suggestion takes advantage of the fact that an electrical half wave transmission line has the same impedance at both ends - it does not matter what the characteristic impedance of the line may be.
>>
>> But unless you have a very long distance to the antenna feedpoint, the loss of practical coax runs is rather low on 40 meters, so the easiest is just to use coax.
>>
>> 73,
>> Don W3FPR
>>
>> On 1/1/2012 12:14 AM, Phil Hystad wrote:
>>> OK, lots of discussion on open wire feeders and other ideas so I thought I would throw out a question to soak up some of the Elecraft community knowledge and wisdom.
>>>
>>> Given a 40-meter dipole antenna, up about 50 feet, what is the best feed configuration for single band usage to minimize loss and minimize common mode currents and radiation from the feed line?
>>>
>>> I am thinking of building such an antenna and I would have no plans for using it on any other band but I would like to use it on the full spectrum of 40 meters with primary focus on the low end for CW.
>>>
>>> My current idea is to cut the antenna for 7.1 MHz but I am not sure about the best plan to feed it. I am thinking of 450 ohm window line (aka ladder line) to a 1:1 balun and the coax the rest of the way into the shack. Or, how about 300 ohm twin lead into a 6:1 balun with coax the rest of the way into the shack? Would this be a significant difference? Or, some other combination?
>>>
>>> Or, maybe a 1:1 balun right at the antenna feed point with coax the remaining distance to the shack?
>>>
>>> Any comments or suggestions?
>>>
>>> I am thinking of also raising up two other dipoles. I have room to put up 80, 40, and 30 as separate dipole antennas -- it is actually a little bit more awkward to do a fan dipole so I am leaning on not doing that.
>>>
>>> 73, phil, K7PEH
>>>
> ______________________________________________________________
> Elecraft mailing list
> Home: http://mailman.qth.net/mailman/listinfo/elecraft
> Help: http://mailman.qth.net/mmfaq.htm
> Post: mailto:[hidden email]
>
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> Please help support this email list: http://www.qsl.net/donate.html
>

Don Wilhelm-4

Re: Single Band 40 Meter Dipole

In reply to this post by Cookie

Willis,

I use two inverted vees mounted at right angles to each other for 80 and
40 meters, no interaction that way.
The feedline may be of interest - it is a Transmission Line Resonator to
make the 80 meter antenna broadbanded - it consists of 1 wavelength of
RG-8 (that includes the balun) followed by a quarter wave of RG-11. The
SWR is below 2:1 except at the very top of 75 meters. You can find the
antenna in the 19th edition of the ARRL Antenna Handbook page 9-16. It
works quite well for me.

I also use 2 fan dipoles - one for 30, 17, and 12 meters, and anther for
20, 15, and 10 meters. I use PVC for spreaders and keep the antenna
wires separated by about 6 inches.

73,
Don W3FPR

On 1/1/2012 12:40 PM, WILLIS COOKE wrote:

> Phil, I would put the balun at the center of the antenna and use one that doubles as a center insulator and attach point for the raising rope. The loss for RG-8X will be about 1 db, so you will not notice. I have had very good luck with attaching an 80 meter dipole to the same coax and balun. I have not tried a 30 meter, but I would expect it to work well also. I have tried a 80-40-20 fan with no good results. I find it best to spread the 80 and 40 at least 30 degrees and probably 90 degrees would be better. You could use a second 75/80 dipole for either the phone or CW band or I have had good results by putting insulators in the 80/75 and using alligator clip leads when I want the longer antenna. The antenna coupler should not be needed for 40 meters or for 80/75 for most of the band is you use the jumpers.
>
> Willis 'Cookie' Cooke
> K5EWJ& Trustee N5BPS, USS Cavalla, USS Stewart
>
>
> ________________________________
> From: Phil Hystad<[hidden email]>
> To: [hidden email]
> Cc: Elecraft List<[hidden email]>
> Sent: Sunday, January 1, 2012 11:13 AM
> Subject: Re: [Elecraft] Single Band 40 Meter Dipole
>
> Don,
>
> goals:
>
> 1. Lowest loss.
> 2. Full 40-meter band coverage but resonant at 7.1 MHz.
> 3. Assume using a tuner (like K3 or KAT500).
> 4. Choke common mode currents.
>
>
> Total feed line length will be on the order of 100 feet for this particular dipole. For the 30 meter dipole I also plan to put up, total feed line length is about 75 feet. My 80 meter dipole will be about 100 feet of feedline.
>
> Before I posted this question to this forum, I would have gone ahead and used the same configuration I used previously when I had my 80 meter doublet up. That antenna was fed by 450 ohm ladder line to a 1:1 balun and then RG-8X to the shack.
>
> But one motivation for my question is weighing the difference in loss of tuner + 450 ohm ladder line and balun versus a direct run of RG8X right to the feed point with a choke balun at that location.
> The RG-8X loss is about 2 dB on that 100 foot run. I could switch to LMR-400 (at 0.7 dB loss) but I am curious about other solutions.
>
> 73, phil, K7PEH
>
>
> On Jan 1, 2012, at 5:39 AM, Don Wilhelm wrote:
>
>> Phil,
>>
>> What do you want to accomplish (other than just an antenna for 40 meters)? Lowest loss? Broadband? Easy- up? Etc? Those parameters will determine your best choice.
>>
>> Easiest IMHO is to just feed it with coax with a current mode choke at the feedpoint. That should present a less than 2:1 SWR over the entire 40 meter band.
>>
>> If you have an extremely long feedline (multiples of a half wave), then you might consider using balanced line cut to some multiple of a half wavelength (electrical length) and the rest of the way to the shack with coax - put the common mode choke at the transition between the balanced line and the coax.
>> This latter suggestion takes advantage of the fact that an electrical half wave transmission line has the same impedance at both ends - it does not matter what the characteristic impedance of the line may be.
>>
>> But unless you have a very long distance to the antenna feedpoint, the loss of practical coax runs is rather low on 40 meters, so the easiest is just to use coax.
>>
>> 73,
>> Don W3FPR
>>
>> On 1/1/2012 12:14 AM, Phil Hystad wrote:
>>> OK, lots of discussion on open wire feeders and other ideas so I thought I would throw out a question to soak up some of the Elecraft community knowledge and wisdom.
>>>
>>> Given a 40-meter dipole antenna, up about 50 feet, what is the best feed configuration for single band usage to minimize loss and minimize common mode currents and radiation from the feed line?
>>>
>>> I am thinking of building such an antenna and I would have no plans for using it on any other band but I would like to use it on the full spectrum of 40 meters with primary focus on the low end for CW.
>>>
>>> My current idea is to cut the antenna for 7.1 MHz but I am not sure about the best plan to feed it. I am thinking of 450 ohm window line (aka ladder line) to a 1:1 balun and the coax the rest of the way into the shack. Or, how about 300 ohm twin lead into a 6:1 balun with coax the rest of the way into the shack? Would this be a significant difference? Or, some other combination?
>>>
>>> Or, maybe a 1:1 balun right at the antenna feed point with coax the remaining distance to the shack?
>>>
>>> Any comments or suggestions?
>>>
>>> I am thinking of also raising up two other dipoles. I have room to put up 80, 40, and 30 as separate dipole antennas -- it is actually a little bit more awkward to do a fan dipole so I am leaning on not doing that.
>>>
>>> 73, phil, K7PEH
>>>
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Bob K6UJ

Re: Single Band 40 Meter Dipole

In reply to this post by Phil Hystad-3

Phil,

LADDERLINE VS COAX
I would use coax feed and a 1:1 balun right where the coax connects to the dipole.
I have used window line and 450 ohm home made ladder line on various antennas over the years and grew tired of working on
feedline issues. "Theoretically" with balanced feedline there will be no radiation from the feedline but the surroundings affect the balance
from the antenna down to the shack. I have used a 1:1 balun as you mentioned at the end of the ladder line and then into the shack with
coax and could solve the RF in the shack issue but the toughest issue to solve was the feedline reradiating electrical noise. The "not so balanced"
feedline attracts noise like a big magnet and reradiates it. Not so good when you are trying to copy weak DX and you have an additional s unit or so of electrical noise.
By experimenting with additional RF choking and grounding as learned from Jim K9YC in his excellent
webpage I could eventually reduce the reradiated noise down to a very low level, however not as low as coax feed with a 1:1 balun at the antenna.
Each new ladder line fed antenna required different RF choking measures. Its not just a hobby, its an adventure, hihihi.

FAN DIPOLE OR SINGLES
I have had very good results with a fan dipole for 80 and 40. The trick is to have enough spacing between the legs of the dipole. My last configuration
had 3 feet separation between the end of the 40 meter dipole and the 80 meter dipole. Much better than lossy traps ! I have tried fan dipoles with
other bands in addition to 80 and 40 and never could get tuned right, one band affects the others, etc.

LOOP ANTENNA ?
I am a big fan of the delta loop antenna. A full size delta for 80 can be used on all bands (80 thru 10) very well with an antenna tuner and depending on the feed point can provide either horizontal or vertical polarization. It needs only a single center support of 50 feet or so and about 100 feet end to end. The lower leg can be zig-zagged to reduce the lower leg dimension without affecting the performance. It is basically a triangle with the apex at the top. It can be "flattened" so the bottom horizontal leg is longer than the other two legs without impairing the effectiveness, actually a flattened delta provides almost a perfect 50 ohm match. It also can be sloped, doesn't have to be strung exactly vertical. The horizontal wire across the bottom can be very low, mine is 6 feet high across the top of a wood fence on the side of my property. A delta gives a nice low radiation angle for DX and has gain over a dipole. It can be fed with either ladder line or coax, gu
ess which one I use ? ( hihi. ) With an antenna tuner an 80 meter delta will cover all bands and will out perform dipoles, particularly on 80 and 40 meters where you would have to have 100 feet or higher dipoles to give a low radiation angle. Comparing the delta to individual dipoles I have found it to be a quieter receive antenna and on 40 and 80 just outstanding, I can copy DX signals on the delta which can not even be heard on the dipole. On the higher bands, above 80/40, unless you have a 70 foot or so tower, the delta will outperform the dipoles. With just a single center support if you have the room for the lower leg it is a great all band consideration.

REFERENCES
I highly recommend these two sources for antenna design and RFI / matching information.
(My ON4UN book is very dog eared. It is my bible on antennas. Read the chapter on loop antennas and you will never be the same. hihi. )

1. Low Band DXing by ON4UN

2. A Ham's Guide to RFI, Ferrites, Baluns, and Audio Interfacing by Jim Brown
available on his webpage http://audiosystemsgroup.com/RFI-Ham.pdf

73,
Bob
K6UJ

On Dec 31, 2011, at 9:14 PM, Phil Hystad wrote: