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Question for Wayne about your OCF Dipole

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marvwheeler

Question for Wayne about your OCF Dipole

Wayne:

In a previous post you mentioned your 90 feet long OCF dipole. Would you
please elaborate on your antenna? Balun, leg lengths, how fed?

Thanks,

Marv

KG7V

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wayne burdick

Re: Question for Wayne about your OCF Dipole

Administrator

Hi Marv,

I didn’t measure the legs. My best guess, standing here by the shack window using an antique brass monocular, is 55’ on one side, 35’ on the other. This is the most satisfying thing about off-center-fed dipoles: length generally isn’t critical. Close your eyes and clip, knowing that, most likely, the ATU will do the rest.

The installation was opportunistic, taking advantage of a back yard gazebo, a Home Depot telescoping pool-cleaning pole, and a huge white oak. Height at each end was determined by how high my son was able to climb without the neighbors noticing.

The experimental feedline, running under the house for half its length, was more deliberate if not justified in the literature. At first I was using 300 ohm mystery twin-lead scavenged from an installation of yore. But given all the RFI sources I decided to try two lengths of LMR-400, taped together every few feet, forming a sortof-balanced-shielded feed. Purists I consulted ahead of time continue to be shocked at how well this works. At the mast, the grounds are left unterminated. In the shack, the two center conductors go to an Elecraft BL2 balun. The shields are soldered together and connected to the BL2’s ground lug.

There you have it: Wide tuning range, low noise, low-loss, and theory-free. How it works is left as an exercise for the reader.

Wayne
N6KR

> On Sep 18, 2017, at 2:31 PM, <[hidden email]> <[hidden email]> wrote:
>
> Wayne:
>
>
>
> In a previous post you mentioned your 90 feet long OCF dipole. Would you
> please elaborate on your antenna? Balun, leg lengths, how fed?
>
>
>
> Thanks,
>
> Marv
>
> KG7V

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Don Sanders

Re: Question for Wayne about your OCF Dipole

So not much different than the G5RV with parallel feed line all the way to
the tuner, no coax.
Just fed off center at the 39% or 61% point instead of center fed.
I wonder about losses in the tuner due to the imbalance but not having
tested it I hold any judgement.
Several articles in the literature of similar OCF antennas fed with
parallel line.
Basically all are non-resonant, so no worries about resonance on higher
bands as with OCF , balun,coax fed antennas.

I used a 88 foot center fed with 2 lengths of RG 6 coax similar to what you
are using.
From Ecuador I worked the world with QRP 40 to 10 meters and with 50 to 100
watts on 80.
Yes, a good antenna. I center fed it due to worries about loss in baluns
with OCF antennas and the lack of commercial balanced line. Just a little
loss from the RG6.

Dr. Don W4BWS
SKCC 81C
K2 SN 163

On Mon, Sep 18, 2017 at 6:02 PM, Wayne Burdick <[hidden email]> wrote:

> Hi Marv,
>
> I didn’t measure the legs. My best guess, standing here by the shack
> window using an antique brass monocular, is 55’ on one side, 35’ on the
> other. This is the most satisfying thing about off-center-fed dipoles:
> length generally isn’t critical. Close your eyes and clip, knowing that,
> most likely, the ATU will do the rest.
>
> The installation was opportunistic, taking advantage of a back yard
> gazebo, a Home Depot telescoping pool-cleaning pole, and a huge white oak.
> Height at each end was determined by how high my son was able to climb
> without the neighbors noticing.
>
> The experimental feedline, running under the house for half its length,
> was more deliberate if not justified in the literature. At first I was
> using 300 ohm mystery twin-lead scavenged from an installation of yore. But
> given all the RFI sources I decided to try two lengths of LMR-400, taped
> together every few feet, forming a sortof-balanced-shielded feed. Purists I
> consulted ahead of time continue to be shocked at how well this works. At
> the mast, the grounds are left unterminated. In the shack, the two center
> conductors go to an Elecraft BL2 balun. The shields are soldered together
> and connected to the BL2’s ground lug.
>
> There you have it: Wide tuning range, low noise, low-loss, and
> theory-free. How it works is left as an exercise for the reader.
>
> Wayne
> N6KR
>
>
>
> > On Sep 18, 2017, at 2:31 PM, <[hidden email]> <
> [hidden email]> wrote:
> >
> > Wayne:
> >
> >
> >
> > In a previous post you mentioned your 90 feet long OCF dipole. Would you
> > please elaborate on your antenna? Balun, leg lengths, how fed?
> >
> >
> >
> > Thanks,
> >
> > Marv
> >
> > KG7V
>
>
>
> ______________________________________________________________
> Elecraft mailing list
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> Post: mailto:[hidden email]
>
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> Message delivered to [hidden email]
>

marvwheeler

Re: Question for Wayne about your OCF Dipole

In reply to this post by wayne burdick

Thanks for your reply. I am going to put something up similar and will comment about my results.

Again, thank and love my K3S=KPA500. I am waiting on UPS to deliver a used KX3 this afternoon.

Thanks for Elecraft support.

73
Marv

-----Original Message-----
From: Wayne Burdick [mailto:[hidden email]]
Sent: Monday, September 18, 2017 3:02 PM
To: [hidden email]
Cc: Elecraft Reflector <[hidden email]>
Subject: Re: [Elecraft] Question for Wayne about your OCF Dipole

Hi Marv,

I didn’t measure the legs. My best guess, standing here by the shack window using an antique brass monocular, is 55’ on one side, 35’ on the other. This is the most satisfying thing about off-center-fed dipoles: length generally isn’t critical. Close your eyes and clip, knowing that, most likely, the ATU will do the rest.

The installation was opportunistic, taking advantage of a back yard gazebo, a Home Depot telescoping pool-cleaning pole, and a huge white oak. Height at each end was determined by how high my son was able to climb without the neighbors noticing.

The experimental feedline, running under the house for half its length, was more deliberate if not justified in the literature. At first I was using 300 ohm mystery twin-lead scavenged from an installation of yore. But given all the RFI sources I decided to try two lengths of LMR-400, taped together every few feet, forming a sortof-balanced-shielded feed. Purists I consulted ahead of time continue to be shocked at how well this works. At the mast, the grounds are left unterminated. In the shack, the two center conductors go to an Elecraft BL2 balun. The shields are soldered together and connected to the BL2’s ground lug.

There you have it: Wide tuning range, low noise, low-loss, and theory-free. How it works is left as an exercise for the reader.

Wayne
N6KR

> On Sep 18, 2017, at 2:31 PM, <[hidden email]> <[hidden email]> wrote:
>
> Wayne:
>
>
>
> In a previous post you mentioned your 90 feet long OCF dipole. Would
> you please elaborate on your antenna? Balun, leg lengths, how fed?
>
>
>
> Thanks,
>
> Marv
>
> KG7V

Bob McGraw - K4TAX

Re: Question for Wayne about your OCF Dipole

In reply to this post by Don Sanders

One very good source of information on OCFD antennas can be found at
http://www.dj0ip.de/antennas/

He also has a very good discussion and examination of various baluns,
common mode chokes, and tuner loss.

73
Bob, K4TAX

On 9/18/2017 6:19 PM, Don Sanders wrote:

> So not much different than the G5RV with parallel feed line all the way to
> the tuner, no coax.
> Just fed off center at the 39% or 61% point instead of center fed.
> I wonder about losses in the tuner due to the imbalance but not having
> tested it I hold any judgement.
> Several articles in the literature of similar OCF antennas fed with
> parallel line.
> Basically all are non-resonant, so no worries about resonance on higher
> bands as with OCF , balun,coax fed antennas.
>
> I used a 88 foot center fed with 2 lengths of RG 6 coax similar to what you
> are using.
> From Ecuador I worked the world with QRP 40 to 10 meters and with 50 to 100
> watts on 80.
> Yes, a good antenna. I center fed it due to worries about loss in baluns
> with OCF antennas and the lack of commercial balanced line. Just a little
> loss from the RG6.
>
> Dr. Don W4BWS
> SKCC 81C
> K2 SN 163
>
>

Don Wilhelm

Re: Question for Wayne about your OCF Dipole

In reply to this post by wayne burdick

Wayne,

Actually, parallel coax runs make a good choice for shielded parallel
feedline. The paralleled LMR-400 lines should produce a feedline
characteristic impedance of 100 ohms. It is normal to connect the
shields together at both ends. At the antenna, the connected shields
are left floating, but at the rig end, are connected to chassis common
(your BL2 ground lug) - perhaps "floating" is what you meant by
"unterminated". Are the shields connected at the antenna end?

As I recall, the matched impedance loss would be twice the loss of a
single run of the coax, but at reasonable distances and the low loss of
LMR-400, it should not be a problem at HF.

The advantages of using parallel coax for a balanced feedline is the
same as using coax over open-wire or ladderline. It can be run on or in
the ground, run next to or thorough conducting metallic surfaces, or
coiled up with no ill effects. Open wire and ladderline takes more care
in its installation and must use gentle turns and be spaced away from
nearby conducting objects.

73,
Don W3FPR

On 9/18/2017 6:02 PM, Wayne Burdick wrote:

>
> The experimental feedline, running under the house for half its length, was more deliberate if not justified in the literature. At first I was using 300 ohm mystery twin-lead scavenged from an installation of yore. But given all the RFI sources I decided to try two lengths of LMR-400, taped together every few feet, forming a sortof-balanced-shielded feed. Purists I consulted ahead of time continue to be shocked at how well this works. At the mast, the grounds are left unterminated. In the shack, the two center conductors go to an Elecraft BL2 balun. The shields are soldered together and connected to the BL2’s ground lug.
>
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Alan Bloom

Re: Question for Wayne about your OCF Dipole

Yes, I agree the shields should be connected together at both ends.

I believe the loss is the same as a single run of coax. Each coax has
only half the loss since it carries only half the power, but there are
two of them so the total loss is the same.

That assumes the SWR is the same in both cases. If 100 ohms is a better
match than 50 ohms, then the loss will be lower in the parallel
configuration.

Alan N1AL

On 09/18/2017 05:31 PM, Don Wilhelm wrote:

> Wayne,
>
> Actually, parallel coax runs make a good choice for shielded parallel
> feedline. The paralleled LMR-400 lines should produce a feedline
> characteristic impedance of 100 ohms. It is normal to connect the
> shields together at both ends. At the antenna, the connected shields
> are left floating, but at the rig end, are connected to chassis common
> (your BL2 ground lug) - perhaps "floating" is what you meant by
> "unterminated". Are the shields connected at the antenna end?
>
> As I recall, the matched impedance loss would be twice the loss of a
> single run of the coax, but at reasonable distances and the low loss of
> LMR-400, it should not be a problem at HF.
>
> The advantages of using parallel coax for a balanced feedline is the
> same as using coax over open-wire or ladderline. It can be run on or in
> the ground, run next to or thorough conducting metallic surfaces, or
> coiled up with no ill effects. Open wire and ladderline takes more care
> in its installation and must use gentle turns and be spaced away from
> nearby conducting objects.
>
> 73,
> Don W3FPR
>
> On 9/18/2017 6:02 PM, Wayne Burdick wrote:
>
>>
>> The experimental feedline, running under the house for half its
>> length, was more deliberate if not justified in the literature. At
>> first I was using 300 ohm mystery twin-lead scavenged from an
>> installation of yore. But given all the RFI sources I decided to try
>> two lengths of LMR-400, taped together every few feet, forming a
>> sortof-balanced-shielded feed. Purists I consulted ahead of time
>> continue to be shocked at how well this works. At the mast, the
>> grounds are left unterminated. In the shack, the two center conductors
>> go to an Elecraft BL2 balun. The shields are soldered together and
>> connected to the BL2’s ground lug.
>>
> ______________________________________________________________
> Elecraft mailing list
> Home: http://mailman.qth.net/mailman/listinfo/elecraft
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> Post: mailto:[hidden email]
>
> This list hosted by: http://www.qsl.net
> Please help support this email list: http://www.qsl.net/donate.html
> Message delivered to [hidden email]
>

wayne burdick

Re: Question for Wayne about your OCF Dipole

Administrator

In reply to this post by Don Wilhelm

> On Sep 18, 2017, at 5:31 PM, Don Wilhelm <[hidden email]> wrote:
>
> Wayne,
>
> Actually, parallel coax runs make a good choice for shielded parallel feedline. The paralleled LMR-400 lines should produce a feedline characteristic impedance of 100 ohms. It is normal to connect the shields together at both ends. At the antenna, the connected shields are left floating, but at the rig end, are connected to chassis common (your BL2 ground lug) - perhaps "floating" is what you meant by "unterminated". Are the shields connected at the antenna end?

Yes.

>
> As I recall, the matched impedance loss would be twice the loss of a single run of the coax, but at reasonable distances and the low loss of LMR-400, it should not be a problem at HF.

Agreed.

>
> The advantages of using parallel coax for a balanced feedline is the same as using coax over open-wire or ladderline. It can be run on or in the ground, run next to or thorough conducting metallic surfaces, or coiled up with no ill effects.

That’s what I was dealing with. The house was built in 1929, so in the crawl space there’s legacy plumbing, knob-and-tube wiring, heating ducts, and spider webs that at least look conductive.

Wayne
N6KR

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Don Wilhelm

Re: Question for Wayne about your OCF Dipole

In reply to this post by Alan Bloom

Alan,

The loss is determined by the current in the coax, not that it carries
1/2 the power. It is the same as for a parallel line - the current is
equal and opposite, so both conductors contribute to the loss.

The total loss should be the same as two runs of coax, but that should
be matched loss. This situation is likely mismatched loss - which
according to the feedline loss with SWR can be relatively small, but
present. Consult the charts in the ARRL handbook and the Antenna
handbook for the loss due to SWR - for reasonable SWR and low loss
feedlines it is minimal. The loss due to SWR is minimized for lines that
have a small matched loss. The specification for LMR-400 is 0.7dB at 30
MHz for 100 feet, so it is small at HF.
In contrast, RG-58 at 10 MHz has 3.6 dB attenuation at 10 MHz per 100 feet.

I may be mistaken (it has happened before), but I am certain that the
loss (in dB) is twice the loss in a single run of coax because the total
length of coax is doubled.

bottom line, with LMR-400 at HF that loss is likely minimal.

For installations that would lead to sharp bends in ladderline or
vintage 300 ohm line or close proximity to conducting surfaces, the use
of parallel coax is a good choice for multiband antennas.

Real open wire may be a better choice if it can be spaced with tension
on the wires and run in spaces that are void of nearby conductors, but
that is difficult to achieve in many situations.
Wayne's "run under the house" may negate the advantages of using
parallel conductor lines and make the parallel coax a better choice.

Your situation may vary, but for low loss coax like LMR-400 at HF, that
may be a good choice for multiband antennas. The only "problem" is the
weight of 2 runs of low loss feedline to the antenna feedpoint. If the
feedpoint is supported by a support pole, then that obstacle is not a
problem.

73,
Don W3FPR

73,
Don W3FPR

On 9/18/2017 9:19 PM, Alan wrote:

> Yes, I agree the shields should be connected together at both ends.
>
> I believe the loss is the same as a single run of coax. Each coax has
> only half the loss since it carries only half the power, but there are
> two of them so the total loss is the same.
>
> That assumes the SWR is the same in both cases. If 100 ohms is a better
> match than 50 ohms, then the loss will be lower in the parallel
> configuration.
>
> Alan N1AL

Buck

Re: Question for Wayne about your OCF Dipole

In reply to this post by wayne burdick

Wouldn't two 50 ohm cables in parallel result in 25 ohm impedance?

k4ia, Buck
K3# 101
Honor Roll 8B DXCC
EasyWayHamBooks.com

On 9/18/2017 10:06 PM, Wayne Burdick wrote:

>
>> On Sep 18, 2017, at 5:31 PM, Don Wilhelm <[hidden email]> wrote:
>>
>> Wayne,
>>
>> Actually, parallel coax runs make a good choice for shielded parallel feedline. The paralleled LMR-400 lines should produce a feedline characteristic impedance of 100 ohms. It is normal to connect the shields together at both ends. At the antenna, the connected shields are left floating, but at the rig end, are connected to chassis common (your BL2 ground lug) - perhaps "floating" is what you meant by "unterminated". Are the shields connected at the antenna end?
>
> Yes.
>
>
>>
>> As I recall, the matched impedance loss would be twice the loss of a single run of the coax, but at reasonable distances and the low loss of LMR-400, it should not be a problem at HF.
>
> Agreed.
>
>
>>
>> The advantages of using parallel coax for a balanced feedline is the same as using coax over open-wire or ladderline. It can be run on or in the ground, run next to or thorough conducting metallic surfaces, or coiled up with no ill effects.
>
> That’s what I was dealing with. The house was built in 1929, so in the crawl space there’s legacy plumbing, knob-and-tube wiring, heating ducts, and spider webs that at least look conductive.
>
> Wayne
> N6KR
>
>
>
> ______________________________________________________________
> Elecraft mailing list
> Home: http://mailman.qth.net/mailman/listinfo/elecraft
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>
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> Message delivered to [hidden email]
>

Charlie T, K3ICH

Re: Question for Wayne about your OCF Dipole

In reply to this post by marvwheeler

I wonder about the power handling capability of some of that cheap RG-6, or whatever they use for TV & satellite.
I've heard of this being done, but can't think of any negatives, except the power rating.
I suspect the actual impedance of the coax is irrelevant.

73, Charlie k3ICH

-----Original Message-----
From: [hidden email] [mailto:[hidden email]] On Behalf Of [hidden email]
Sent: Monday, September 18, 2017 7:53 PM
To: 'Wayne Burdick'
Cc: 'Elecraft Reflector'
Subject: Re: [Elecraft] Question for Wayne about your OCF Dipole

Thanks for your reply. I am going to put something up similar and will comment about my results.

Again, thank and love my K3S=KPA500. I am waiting on UPS to deliver a used KX3 this afternoon.

Thanks for Elecraft support.

73
Marv

-----Original Message-----
From: Wayne Burdick [mailto:[hidden email]]
Sent

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Jim Brown-10

Re: Question for Wayne about your OCF Dipole

In reply to this post by wayne burdick

On 9/18/2017 7:06 PM, Wayne Burdick wrote:
>> As I recall, the matched impedance loss would be twice the loss of a single run of the coax, but at reasonable distances and the low loss of LMR-400, it should not be a problem at HF.
> Agreed.

The matched loss in parallel runs of coax, whether wired in series (to
double the Zo) or in parallel (to divide Zo by two), is the same as for
a single run of the same coax.

The virtues of parallel runs are the lack of interference with
surrounding objects and the impedance transformation of the series or
parallel connection.

73, Jim K9YC

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Barry K3NDM

Re: Question for Wayne about your OCF Dipole

In reply to this post by Don Wilhelm

Fellas,
I think this subject is being over thought. An OCFD is nothing more
than a dipole cut for the lowest operating frequency with an off center
feed. By doing this, it allows reasonable match for multiples of the
fundamental frequency.

If a center fed dipole is ~50-75 Ohms at say 80 meters, it will be
somewhere around 700-1000 Ohms on even multiples. Feeding the antenna
off the center just puts the feed at a point that reflects a more
reasonable impedance. At this point there are two ways to go, the
Carolina Windom way or the straight OCFD. I have used both
configurations and they do really work well if you can get the antenna
up in the clear. Feeding it with something like LMR-400 keeps the losses
reasonable even with an SWR that may be 4 or 5:1. In both cases a 4:1
balun should be used, a current type if you use the simpler OCFD. Every
Elecraft tuner I have used handles this without breaking a sweat. Happy
DXing!

73,
Barry
K3NDM

------ Original Message ------
From: "Don Wilhelm" <[hidden email]>
To: [hidden email]
Sent: 9/18/2017 10:20:22 PM
Subject: Re: [Elecraft] Question for Wayne about your OCF Dipole

>Alan,
>
>The loss is determined by the current in the coax, not that it carries
>1/2 the power. It is the same as for a parallel line - the current is
>equal and opposite, so both conductors contribute to the loss.
>
>The total loss should be the same as two runs of coax, but that should
>be matched loss. This situation is likely mismatched loss - which
>according to the feedline loss with SWR can be relatively small, but
>present. Consult the charts in the ARRL handbook and the Antenna
>handbook for the loss due to SWR - for reasonable SWR and low loss
>feedlines it is minimal. The loss due to SWR is minimized for lines
>that have a small matched loss. The specification for LMR-400 is 0.7dB
>at 30 MHz for 100 feet, so it is small at HF.
>In contrast, RG-58 at 10 MHz has 3.6 dB attenuation at 10 MHz per 100
>feet.
>
>I may be mistaken (it has happened before), but I am certain that the
>loss (in dB) is twice the loss in a single run of coax because the
>total length of coax is doubled.
>
>bottom line, with LMR-400 at HF that loss is likely minimal.
>
>For installations that would lead to sharp bends in ladderline or
>vintage 300 ohm line or close proximity to conducting surfaces, the use
>of parallel coax is a good choice for multiband antennas.
>
>Real open wire may be a better choice if it can be spaced with tension
>on the wires and run in spaces that are void of nearby conductors, but
>that is difficult to achieve in many situations.
>Wayne's "run under the house" may negate the advantages of using
>parallel conductor lines and make the parallel coax a better choice.
>
>Your situation may vary, but for low loss coax like LMR-400 at HF, that
>may be a good choice for multiband antennas. The only "problem" is the
>weight of 2 runs of low loss feedline to the antenna feedpoint. If the
>feedpoint is supported by a support pole, then that obstacle is not a
>problem.
>
>73,
>Don W3FPR
>
>73,
>Don W3FPR
>
>
>On 9/18/2017 9:19 PM, Alan wrote:
>>Yes, I agree the shields should be connected together at both ends.
>>
>>I believe the loss is the same as a single run of coax. Each coax has
>>only half the loss since it carries only half the power, but there are
>>two of them so the total loss is the same.
>>
>>That assumes the SWR is the same in both cases. If 100 ohms is a
>>better match than 50 ohms, then the loss will be lower in the parallel
>>configuration.
>>
>>Alan N1AL
>______________________________________________________________
>Elecraft mailing list
>Home: http://mailman.qth.net/mailman/listinfo/elecraft
>Help: http://mailman.qth.net/mmfaq.htm
>Post: mailto:[hidden email]
>
>This list hosted by: http://www.qsl.net
>Please help support this email list: http://www.qsl.net/donate.html
>Message delivered to [hidden email]

Jim Brown-10

Re: Question for Wayne about your OCF Dipole

In reply to this post by Charlie T, K3ICH

On 9/18/2017 8:17 PM, Charlie T wrote:
> I wonder about the power handling capability of some of that cheap RG-6, or whatever they use for TV & satellite.

CATV coax is optimized for physical properties and for RF performance at
UHF (CATV does include "backhaul" information from the customer for
billing, etc., and that does run at HF).

Nearly all CATV coax has a copper-clad steel center and a shield with
braid and one or more foils. The shield is usually 100% Al. Loss in coax
below UHF is all due to I squared R losses. The steel center causes
increased loss at MF and low HF, while skin effect takes the steel out
of the picture at VHF and above. And because it's Al, shield resistance
is higher than for a good copper braid.

Further, an important property of a cable shield is its transfer
impedance, which is the ratio of differential voltage induced by current
on the outside of the shield. In other words, its a parameter defining
the effectiveness of a shield against common mode current. The lower the
transfer impedance, the better the shielding, and the lower limit on
transfer impedance is the resistance of the shield at the frequency of
interest. That's an important reason for using common mode chokes on
coax used for RX antennas!

W8JI says that "RG6 cables can handle TX power," without qualifying
which RG6 cables he's talking about (there are hundreds of them). I use
CATV RG6 extensively for RX antennas, and buy flooded Commscope on 1,000
ft spools for about $95.

While thinking about OCF dipoles, remember that because they are
inherently unbalanced, they tend to pick up lots of noise on the
feedline, and because they're nearly always fed with 2-wire line and
usually a poor match on most bands, they are impractical to choke
effectively. [To understand why, study my tutorial k9yc.com/RFI-Ham.pdf]
No problem if you're in a quiet location in the middle of nowhere, but
not a great choice if, like most of us, you're surrounded by neighbors,
each with dozens of RF noise sources.

73, Jim K9YC

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Don Wilhelm

Re: Question for Wayne about your OCF Dipole

In reply to this post by Charlie T, K3ICH

Charlie,

The power handling efficiency of RG-6 is better than equal size RG-8X,
so it should work well. Look at the maximum voltage ratings of both.

73,
Don W3FPR

On 9/18/2017 11:17 PM, Charlie T wrote:
> I wonder about the power handling capability of some of that cheap RG-6, or whatever they use for TV & satellite.
> I've heard of this being done, but can't think of any negatives, except the power rating.
> I suspect the actual impedance of the coax is irrelevant.
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Vic Rosenthal

Re: Question for Wayne about your OCF Dipole

In reply to this post by Jim Brown-10

With any OCF or similar antenna there will be some bands on which the SWR on the coax is very high. And if you are trying to operate it at frequencies below its halfwave resonance, it can be astronomical.
I'm running a 10m long rotatable dipole, center fed, on all bands from 40 through 6m. There is no way this could work with any kind of coax feed; the SWR on 40m is around 100:1.
The feedline is 34 feet (about 10m long). I discovered that the losses in a balun at the rig end are too great -- I nearly destroyed a 5kw rated DXE balun trying to do this (power was 1.2 kW). You need either a balanced tuner, or some balanced arrangement to cancel the reactance before the balun. I chose the latter.
I recently upgraded my balanced feedline to no. 12 spaced 10 cm. I put a twist in it about every 1m to improve balance. Calculated loss on 40m (by far the worst band) is less than 2 dB.

Vic 4X6GP

> On 19 Sep 2017, at 6:57, Jim Brown <[hidden email]> wrote:
>
>> On 9/18/2017 8:17 PM, Charlie T wrote:
>> I wonder about the power handling capability of some of that cheap RG-6, or whatever they use for TV & satellite.
>
> CATV coax is optimized for physical properties and for RF performance at UHF (CATV does include "backhaul" information from the customer for billing, etc., and that does run at HF).
>
> Nearly all CATV coax has a copper-clad steel center and a shield with braid and one or more foils. The shield is usually 100% Al. Loss in coax below UHF is all due to I squared R losses. The steel center causes increased loss at MF and low HF, while skin effect takes the steel out of the picture at VHF and above. And because it's Al, shield resistance is higher than for a good copper braid.
>
> Further, an important property of a cable shield is its transfer impedance, which is the ratio of differential voltage induced by current on the outside of the shield. In other words, its a parameter defining the effectiveness of a shield against common mode current. The lower the transfer impedance, the better the shielding, and the lower limit on transfer impedance is the resistance of the shield at the frequency of interest. That's an important reason for using common mode chokes on coax used for RX antennas!
>
> W8JI says that "RG6 cables can handle TX power," without qualifying which RG6 cables he's talking about (there are hundreds of them). I use CATV RG6 extensively for RX antennas, and buy flooded Commscope on 1,000 ft spools for about $95.
>
> While thinking about OCF dipoles, remember that because they are inherently unbalanced, they tend to pick up lots of noise on the feedline, and because they're nearly always fed with 2-wire line and usually a poor match on most bands, they are impractical to choke effectively. [To understand why, study my tutorial k9yc.com/RFI-Ham.pdf] No problem if you're in a quiet location in the middle of nowhere, but not a great choice if, like most of us, you're surrounded by neighbors, each with dozens of RF noise sources.
>
> 73, Jim K9YC
>
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Edward R Cole

Re: Question for Wayne about your OCF Dipole

In reply to this post by marvwheeler

A few years ago, at a previous home with 5 ac, I considered an 80m
full wave horizontal loop fed with ladder line (450-ohm) but coming
thru the house wall would require hefty insulators so I considered
running parallel coax as Wayne described and have an parallel tuner
on the wall. Theoretical loop Z is 140-ohms so I was thinking
parallel RG-11.

Still have the big variable caps needed for the tuner, but I bought a
old Drake MN2000 and now have the KXPA100 with atu for the tribander
and inverted-V; Inverted-L has a tapped base coil.

Well, short story: "in 2006 we moved to property with heavily
forested 1-3/4 ac so loop idea was abandoned and I put up an
inverted-L between two towers separated 130 foot".

73, Ed - KL7UW
http://www.kl7uw.com
Dubus-NA Business mail:
[hidden email]

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k6dgw

Re: Question for Wayne about your OCF Dipole

In reply to this post by Buck

They're in series when used in the manner being discussed, not
parallel. If you were to connect the two center conductors together and
two shields together at each end, they are in parallel. In such case,
the shields go to one side of the dipole and the center conductors to
the other side.

On 9/18/2017 8:14 PM, Buck wrote:
> Wouldn't two 50 ohm cables in parallel result in 25 ohm impedance?
>
>
> k4ia, Buck
> K3# 101
> Honor Roll 8B DXCC
> EasyWayHamBooks.com

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Bob McGraw - K4TAX

Re: Question for Wayne about your OCF Dipole

Most HF antennas for 160M, 80M and 40M, based on average height above ground, have a feed Z less than 50 ohms at resonance. Some as low as 20 ohms.

Bob, K4TAX

Sent from my iPhone

> On Sep 19, 2017, at 12:47 PM, Fred Jensen <[hidden email]> wrote:
>
> They're in series when used in the manner being discussed, not parallel. If you were to connect the two center conductors together and two shields together at each end, they are in parallel. In such case, the shields go to one side of the dipole and the center conductors to the other side.
>
>> On 9/18/2017 8:14 PM, Buck wrote:
>> Wouldn't two 50 ohm cables in parallel result in 25 ohm impedance?
>>
>>
>> k4ia, Buck
>> K3# 101
>> Honor Roll 8B DXCC
>> EasyWayHamBooks.com
>
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Jim Brown-10

Re: Question for Wayne about your OCF Dipole

On 9/19/2017 10:55 AM, Bob McGraw K4TAX wrote:
> Most HF antennas for 160M, 80M and 40M, based on average height above ground, have a feed Z less than 50 ohms at resonance. Some as low as 20 ohms.

Yes, but that depends strongly on ground conductivity and electrical
height. I'm part of a team that activates CA counties with few if any
hams for the CA QSO Party, and do most of the antenna design work. The
QTHs where we set up mostly have very poor ground, and I take that into
account when modeling them in NEC. Our 40M dipole for the SSB station
was rigged from trees at about 35 ft, and modeling predicted a feedpoint
Z around 75 ohms, so we fed it with RG11 (it was a long run to minimize
QRM to/from the CW station on 40M). When rigged, I checked SWR with an
AEA SWR bridge calibrated to 75 ohms and the SWR was close to 1:1. That
antenna at the same height over good ground modeled closer to 50 ohms.

Using a vector network analyzer and SimSmith software to transform
measurements in the shack to the feedpoint, I've measured the feedpoint
Z of my 120 - 140 ft high 80M dipoles over poor ground in the range of
85-90 ohms.

Feedpoint Z of horizontal antennas includes the mutual impedance of the
ground reflection, and that reflection varies in both magnitude and
phase with electrical height and ground conductivity. This can be
clearly seen in graphs of computed feedpoint Z vs height for horizontal
antennas, which show low dipoles oscillating around 50 ohms and high
dipoles oscillating around 75 ohms. One place I recall seeing such
graphs is in ON4UN's Low Band DXing, much of which is about antennas and
counterpoise/radial systems for 40-160M.

73, Jim K9YC

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