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Author Topic: Feeding Fan Dipole w/ Ladder Line  (Read 6666 times)
W5DXP
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« Reply #15 on: June 11, 2013, 08:40:22 AM »

... it is 1/3 the loss, but the total amount of loss is insignificant in any case.

Dale, I guess I'm just a myth buster wannabe. We can argue over the definition of "insignificant" but not over the definition of "most efficient". The question is: Is the following statement true or false?

"If you're feeding resonant antennas that have a 50-ohm feed point, that would mean 50-ohm coax would be the most efficient path between your radio and the antenna."
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G8JNJ
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« Reply #16 on: June 11, 2013, 08:57:39 AM »

Hi Dave,

You stated that

"Advantage of a fan dipole fed with ladder line

A dipole exhibits multiple azimuth lobes on frequencies where the length exceeds 1/2 wavelength.

A number of 1/2 wavelength dipoles fed together will exhibit a nice figure-8 azimuth pattern on every band for which there is a 1/2 wavelength dipole.

A fan dipole having wires for 80, 20, 15, and 10 meters, and fed with ladder line, should make a fine all-band antenna. I can model this in EZNEC if you'd like"

I'm interested to see if this is true. What happens on 10MHz, 18MHz & 24MHz where the 3.5MHz dipole is near to being an odd multiple of a wavelength long. Surely you would still get multiple lobes forming.

If you are feeding a fan type antenna with open wire and want it to look more like a dipole on each band. Wouldn't it be better to use a skeleton bowtie or bicone type shape made from multiple wires. Which has natural wide bandwidth and a feedpoint impedance closer to that of open line ?

Regards,

Martin - G8JNJ

www.g8jnj.webs.com


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WB6BYU
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« Reply #17 on: June 11, 2013, 09:57:48 AM »

Quote from: W5DXP
Quote from: WB6BYU
... it is 1/3 the loss, but the total amount of loss is insignificant in any case.

Dale, I guess I'm just a myth buster wannabe. We can argue over the definition of "insignificant" but not over the definition of "most efficient". The question is: Is the following statement true or false?

"If you're feeding resonant antennas that have a 50-ohm feed point, that would mean 50-ohm coax would be the most efficient path between your radio and the antenna."



I know, it's tough to be technically correct without becoming far too verbose to get
all the conditions properly stated!

I agree that the statement is not ALWAYS true, because it is possible to choose conditions
where it isn't (as you did).  On the other hand, the examples I gave at 28.5 MHz the coax
(particularly choosing a low-loss type such as LMR-400) actually had slightly LESS loss than
the open wire line (again, depending on the exact parameters.) 

Choosing the specific materials on hand (RG-8X and #14 copper to make open wire line)
with a 50 ohm load at 28.5 MHz (where losses will be higher) the calculated numbers are
1.6dB loss for coax and about 0.7dB for open wire line plus about 0.1 to 0.3dB loss in the
tuner depending on the electrical length of the line (assuming a lossless 1 : 1 balun.)

On 80m the corresponding losses are about 0.57dB vs. 0.25dB + 0.3dB tuner loss.  At that
point we're splitting hairs.

Meanwhile, on 80m (and more-so on 160m) we have to consider the skin effect losses if the
open wire line is made using Copperweld(R), which VK1OD's calculator doesn't take into account,
as well as the losses in the balun, tuner, how the feedline is routed with respect to brick
walls, whether the insulation is removed from the copper wire, etc.

So the best we can say in general is that the losses are similar, while providing specific
examples where it could go one way or the other.
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KK4CRY
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« Reply #18 on: June 11, 2013, 10:41:41 AM »

it will most likely be in a Invert V orientation, this is most likely the only antenna i will be able to raise unless i can convince otherwise
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W5DXP
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« Reply #19 on: June 12, 2013, 07:35:46 AM »

I agree that the statement is not ALWAYS true, because it is possible to choose conditions where it isn't (as you did).

In fact, changing "would" to "could" in the original assertion changes it from a false (exclusive) statement to a true (inclusive) statement. Here's what I am saying in technical terms: The maximum power transfer theorem applied to RF antenna systems is just as valid for non-resonant, mismatched antennas as it is for resonant matched antennas.
« Last Edit: June 12, 2013, 07:43:52 AM by W5DXP » Logged
N4JTE
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« Reply #20 on: June 12, 2013, 03:21:28 PM »

If I may summarize, hopefully accurately, there is no discernable advantage to using ladderline to a fan dipole instead of direct 50 ohm coax.
bob
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WX7G
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« Reply #21 on: June 12, 2013, 05:23:14 PM »

Hi Dave,

You stated that

"Advantage of a fan dipole fed with ladder line

A dipole exhibits multiple azimuth lobes on frequencies where the length exceeds 1/2 wavelength.

A number of 1/2 wavelength dipoles fed together will exhibit a nice figure-8 azimuth pattern on every band for which there is a 1/2 wavelength dipole.

A fan dipole having wires for 80, 20, 15, and 10 meters, and fed with ladder line, should make a fine all-band antenna. I can model this in EZNEC if you'd like"

I'm interested to see if this is true. What happens on 10MHz, 18MHz & 24MHz where the 3.5MHz dipole is near to being an odd multiple of a wavelength long. Surely you would still get multiple lobes forming.

If you are feeding a fan type antenna with open wire and want it to look more like a dipole on each band. Wouldn't it be better to use a skeleton bowtie or bicone type shape made from multiple wires. Which has natural wide bandwidth and a feedpoint impedance closer to that of open line ?

Regards,

Martin - G8JNJ

www.g8jnj.webs.com


Martin, you are right! Even with an additional dipole for 7 MHz the 3.5 MHz dipole hogs the current on the WARC bands and results in a radiation pattern that is not the figure-8 of a dipole.
« Last Edit: June 12, 2013, 05:25:17 PM by WX7G » Logged
W5DXP
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« Reply #22 on: June 12, 2013, 07:39:40 PM »

If I may summarize, hopefully accurately, there is no discernable advantage to using ladderline to a fan dipole instead of direct 50 ohm coax.

 Wireman #551 ladder-line, $33 per 100 ft. Smiley
 Wireman #110 RG-213 coax, $80 per 100 ft. Sad
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N4JTE
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« Reply #23 on: June 12, 2013, 07:46:01 PM »

Cecil you forgot to add in cost of the tuner, hi
bob
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W5DXP
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« Reply #24 on: June 12, 2013, 07:50:27 PM »

Cecil you forgot to add in cost of the tuner, hi

This would work just as well with a fan dipole. Smiley

http://www.w5dxp.com/notuner.htm
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G8JNJ
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« Reply #25 on: June 13, 2013, 01:40:16 AM »

Hi Dave,

WRT your comment.

"Martin, you are right! Even with an additional dipole for 7 MHz the 3.5 MHz dipole hogs the current on the WARC bands and results in a radiation pattern that is not the figure-8 of a dipole"

Now try modelling a simple flat 3 wire bicone with traditional 'kite' shaped elements.

Longest wire of each half of say 10m. Boundary / outside wire each side spaced at 7m length by 2m from the 10m long centre wire. All three wires forming the kite shape are joined at both ends. Suspend horizontally so the wires are parallel to the ground at a height of about 10 to 20m AGL.

Feed with 200 Ohm line and 4:1 balun.

Advantages

- Very wide bandwidth
- Lowest frequency of operation is significantly lower than that of an equivalent length dipole
- 3:1 SWR at lowest resonance to highest frequencies > 50MHz. Occasionally peaking 5:1
- Reduced size of impedance excursions at feed point so easy to match with minimal losses
- Current distribution at higher frequencies tends towards a travelling wave component. So lobes become partially filled

Disadvantages

- Very slightly worse looking than a fan dipole and requires suspending horizontally
- Still needs a tuner on some frequencies

Drop me an email if you wish to discuss further.

Regards,

Martin - G8JNJ

www.g8jnj.webs.com




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G8JNJ
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« Reply #26 on: June 13, 2013, 06:10:01 AM »

Correction last post should have said

- Nominally 1.5 to 3:1 SWR at lowest resonance to highest frequencies > 50MHz. Occasionally may peak up to Max of 5:1
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N4HRA
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« Reply #27 on: June 15, 2013, 09:22:28 AM »

I have been running a dipole with ladder line to  a SGC remote ATU for years tuning 10 - 80M no issue.
I have the ladder line from the Dipole connected directly to a remote ATU outside.
then coax from the ATU to the rig
I use a Base Tee to supply 12Vdc across the coax to power the ATU
The nice thing about the SGC, no tune command line is required.
Set the rig to 10W CW and key the rig.

Just my 2 cents

Lew
N4HRA
 Grin


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W5DXP
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« Reply #28 on: June 15, 2013, 09:43:30 AM »

... then coax from the ATU to the rig

Have you ever measured the ladder-line current balance or the common-mode current on that coax? Given that the coax braid is tied directly to one wire of the balanced ladder-line, there should be considerable unbalance in the ladder-line currents and lots of feedline radiation. The schematic would look something like this:

Coax center conductor---tuner network---Ladder-Line wire#1--------

Coax braid conductor----tuner ground-----Ladder-Line wire#2-------
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N4HRA
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« Reply #29 on: June 15, 2013, 11:39:04 AM »

No I have not measured the ladder-line current balance or the common-mode current on that coax.

SGC Smart tuners for Stealth Antennas show several method of using the tuner
Vertical, End Fed Wire, or Marconi, Inverted L, Center Fed, Loop

SGC call it a coupler: Antenna couplers are placed at the antenna, and precisely match conditions of
the antenna to the feed line.

and from what I have read the ladder line is part of the antenna

Enough said

Again just my 2 cents

Lew

N4HRA
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