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Author Topic: Any Experience - Input? Coax / Ladder Line Conversion Kit  (Read 10195 times)
WB6BYU
Member

Posts: 12974




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« Reply #15 on: October 05, 2013, 07:23:29 AM »

Quote from: WH7DX

You would think that if you have a single line of power and it split into 3 or so sections/bands and was radiating energy - the energy to the primary band (frequency working) would be diminished or that the other non-resonant bands would we affecting it some how - power loss etc. 




Assume for the moment you have a set of dipoles for 80m, 40m and 20m, and feed
power to it on 40m.  What happens to the power that goes into the dipoles for the
other bands?   Any current flowing on the other wires is still in phase with that
on the desired wire, so any radiation from those wires will reinforce the desired
dipole.  (This isn't always the case when one of the wires is longer than 1/2
wavelength.)  There isn't any significant resistance or loss involved, so, other
than a slight shift in pattern, the antenna would still radiate the power supplied
to it.

But the current does NOT divide evenly among the wires.  Consider the case
where you have resistors in parallel:  50 ohms and 2000 ohms.  The total
resistance is ~49 ohms.  If you applied 100 volts across the pair of them,
2 amps would flow though the 50 ohm resistor and only 50mA in the 2K
resistor.  For a first-order analysis of the circuit you could ignore the large
resistor, as little current flows through it.

The same is true of multiple antennas on the same feedpoint:  the current
divides among them based on the relative impedances at the operating
frequency.  In this case, the 80m dipole might look like 2000 ohms and
the 20m dipole might look like 12-j1000 ohms.  In both cases the
impedance is high (due to the reactance in the case of the 20m wire) so
relatively little current flows on the wire.

Where this falls apart is when you have more than one set of wires with
a low impedance, or where the impedances interact (as may be the case
when the wires are close to the same resonant frequency.)


This combination is commonly called a "fan dipole" these days, though I
still call it "multiple dipoles on a common feedpoint", since the traditional
fan dipole is a different antenna.  (A true fan dipole can cover a wide
bandwith, such as 40m through 10m continuously with an SWR under
3 : 1 when using a 300 ohm feed.  By contrast, multiple dipoles on a
common feedpoint may cover multiple narrow frequency ranges across
that span.)
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W5DXP
Member

Posts: 3527


WWW

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« Reply #16 on: October 05, 2013, 08:20:53 AM »

I'm thinking about the G5RV 102ft version with possible 80m but 15-40m is my real target - and 15-20m critical or won't do it.

Here's what EZNEC says about the SWR on the coax at the coax/twinlead junction of a 102' G5RV.

Freq, SWR50, SWR75
14.2, 2.0:1, 1.4:1
18.14, 27:1, 19:1
21.3, 16:1, 12:1

Seems to me that the performance of a 102' G5RV will be a big disappointment on 15m and 17m. It is a pretty good antenna on 80m, 40m, 20m, and 12m but I wouldn't recommend it for your requirements. A fan or trapped dipole might be a better choice.



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73, Cecil, www.w5dxp.com
The purpose of an antenna tuner is to increase the current through the radiation resistance at the antenna to the maximum available magnitude resulting in a radiated power of I2(RRAD) from the antenna.
WH7DX
Member

Posts: 1029




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« Reply #17 on: October 05, 2013, 11:26:51 AM »

Quote from: WH7DX

You would think that if you have a single line of power and it split into 3 or so sections/bands and was radiating energy - the energy to the primary band (frequency working) would be diminished or that the other non-resonant bands would we affecting it some how - power loss etc. 




Assume for the moment you have a set of dipoles for 80m, 40m and 20m, and feed
power to it on 40m.  What happens to the power that goes into the dipoles for the
other bands?   Any current flowing on the other wires is still in phase with that
on the desired wire, so any radiation from those wires will reinforce the desired
dipole.  (This isn't always the case when one of the wires is longer than 1/2
wavelength.)  There isn't any significant resistance or loss involved, so, other
than a slight shift in pattern, the antenna would still radiate the power supplied
to it.

But the current does NOT divide evenly among the wires.  Consider the case
where you have resistors in parallel:  50 ohms and 2000 ohms.  The total
resistance is ~49 ohms.  If you applied 100 volts across the pair of them,
2 amps would flow though the 50 ohm resistor and only 50mA in the 2K
resistor.  For a first-order analysis of the circuit you could ignore the large
resistor, as little current flows through it.

The same is true of multiple antennas on the same feedpoint:  the current
divides among them based on the relative impedances at the operating
frequency.  In this case, the 80m dipole might look like 2000 ohms and
the 20m dipole might look like 12-j1000 ohms.  In both cases the
impedance is high (due to the reactance in the case of the 20m wire) so
relatively little current flows on the wire.

Where this falls apart is when you have more than one set of wires with
a low impedance, or where the impedances interact (as may be the case
when the wires are close to the same resonant frequency.)


This combination is commonly called a "fan dipole" these days, though I
still call it "multiple dipoles on a common feedpoint", since the traditional
fan dipole is a different antenna.  (A true fan dipole can cover a wide
bandwith, such as 40m through 10m continuously with an SWR under
3 : 1 when using a 300 ohm feed.  By contrast, multiple dipoles on a
common feedpoint may cover multiple narrow frequency ranges across
that span.)


Thank again for the info Dale.  Everything really helped in my seeing what would work best for any future antenna I might do AND I'm going to redo my current dipole configuration FIRST - getting a push up mast I think - and get my dipoles up higher and use a fan style dipole with 40 and 80 on one coax.   That's a cleaner setup and better than putting a single dipole below another. 

W5DXP - Yes - that wouldn't work out very well.   Thank you.
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AA4HA
Member

Posts: 1377




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« Reply #18 on: October 05, 2013, 12:54:53 PM »

If you can make your own ladder line you will get slightly better performance than the foam/plastic dielectric commercial stuff. The longest run of ladder line I have is 600 feet long (I love wire antennas).  You do want to keep it off of the ground, I keep it high enough up off the ground so I do not get "clotheslined" while riding on an ATV. Most commercial AM stations still use ladder line and some of their runs are very long (see links to pictures, they look like power lines);

http://gallery.bostonradio.org/2002-03/wsm/100-00497-med.html
http://gallery.bostonradio.org/2002-03/wsm/100-00498-med.html

Tisha
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Ms. Tisha Hayes, AA4HA
Lookout Mountain, Alabama
WH7DX
Member

Posts: 1029




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« Reply #19 on: October 05, 2013, 01:35:42 PM »

If you can make your own ladder line you will get slightly better performance than the foam/plastic dielectric commercial stuff. The longest run of ladder line I have is 600 feet long (I love wire antennas).  You do want to keep it off of the ground, I keep it high enough up off the ground so I do not get "clotheslined" while riding on an ATV. Most commercial AM stations still use ladder line and some of their runs are very long (see links to pictures, they look like power lines);

http://gallery.bostonradio.org/2002-03/wsm/100-00497-med.html
http://gallery.bostonradio.org/2002-03/wsm/100-00498-med.html

Tisha

That's the problem - I can't keep the whole run off the ground...
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AA4PB
Member

Posts: 12638




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« Reply #20 on: October 05, 2013, 01:38:39 PM »

If you can't keep the feedline off the ground then you shouldn't use ladderline.
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WH7DX
Member

Posts: 1029




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« Reply #21 on: October 05, 2013, 05:23:37 PM »

Roger... I've kinda worked through that and I'm back to my normal coax - LMR400...   TU

I didn't know the problem with ladder on the ground.. too bad...
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WB6BYU
Member

Posts: 12974




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« Reply #22 on: October 05, 2013, 06:04:09 PM »

Can you run the ladder line inside some 6" or larger diameter drain pipe?
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WH7DX
Member

Posts: 1029




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« Reply #23 on: October 05, 2013, 07:44:46 PM »

Can you run the ladder line inside some 6" or larger diameter drain pipe?

No I can't - but it doesn't matter.   When I look at everything - using something like LMR400 with a multi-band dipole that can handle 700-800W is the way to go for me.   The coax will last longer and isn't as sensitive, the cost is comparable and SWR shouldn't be an issue with 40-80M and even 20M should be fine.   I'm not sure I'm going to do this - but I'm learning what's involved.   I'll need to see if the location if worth the effort now.

QUESTION:   With my single 40M, 80M and 160M dipoles - each feed separately - I was told that I didn't need a balun because the dipoles were balanced - by DX Engineering - I believe they are the Alpha-Delta Center connectors with end isolators etc.    I was using the Palomar Ferrite Chokes on the 40 and 80 and double for the 160.   I'm hearing difference stories?     And does it matter if a single coax is feeding a 40 and 80m dipole.


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K3VAT
Member

Posts: 699




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« Reply #24 on: October 06, 2013, 05:01:03 AM »

...
QUESTION:   With my single 40M, 80M and 160M dipoles - each feed separately - I was told that I didn't need a balun because the dipoles were balanced - by DX Engineering - I believe they are the Alpha-Delta Center connectors with end isolators etc.    I was using the Palomar Ferrite Chokes on the 40 and 80 and double for the 160.   I'm hearing difference stories?     And does it matter if a single coax is feeding a 40 and 80m dipole. 

I recommend that you start here to understand the functions of balun/chokes: http://w8ji.com/; there are also excellent articles on this website about a few of the other technical points that have been discussed in this thread.

Accurate test and measurement equipment IS the way to see how your antenna system is functioning; no room for opinions.

GL, 73, Rich, K3VAT
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W5DXP
Member

Posts: 3527


WWW

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« Reply #25 on: October 06, 2013, 07:08:06 AM »

I was using the Palomar Ferrite Chokes on the 40 and 80 and double for the 160.   I'm hearing difference stories?

It is good engineering practice to install a balun at every BALanced to UNbalanced junction in an antenna system, e.g. a BALanced dipole fed with UNbalanced coax. A ferrite choke IS a w2du balun (See The ARRL Handbook). Some hams do not follow good engineering practice and get away with it for reasons that they may or may not understand, e.g. feedline length has a large effect.
Logged

73, Cecil, www.w5dxp.com
The purpose of an antenna tuner is to increase the current through the radiation resistance at the antenna to the maximum available magnitude resulting in a radiated power of I2(RRAD) from the antenna.
WB6BYU
Member

Posts: 12974




Ignore
« Reply #26 on: October 06, 2013, 08:18:03 AM »

Quote from: WH7DX

...I was told that I didn't need a balun because the dipoles were balanced...



Do you need a balun?  No.  Many hams use dipoles without them and don't notice
adverse effects that they associate with the lack of a balun.

Is it good practice?  Yes.  It ensures that your antenna works in the way you expect. 
This is true regardless of the number of dipole wires you put on a single feedpoint.

The balun is supposed to prevent common mode current on the feedline.  How much
common mode current there is without a balun depends on a lot of factors, including
the length of the feedline (relative to frequency) and what else is connected to it
in the shack.  Without a balun you may find that the receive noise level is higher,
or the SWR changes when you plug a cable into an unused port on an antenna switch,
or you get RF into your mic circuit.  (In my case my keyer continues sending after
I let off the paddles.)  Most hams don't recognize these as due to lack of a balun, and
sometimes they can be cured by grounding the station or changing the length of the
feedline.

The balun makes your antenna performance more consistent, independent of what
happens to the coax after it leaves the antenna.


Now I should also clarify that in this context "balun" doesn't necessarily mean a
cylindrical object connected between the antenna wires and the coax.
  That is
one physical form, but there are other manifestations.  The ferrite chokes on the
coax, for example, are current baluns because they block (or at least are intended
to reduce) the RF current on the outside of the coax shield - which is what we mean
by "common mode current".  Similarly, ground the shield of the coax 1/4 wavelength
down from the antenna does the same thing (though it doesn't work as well for a
multiband antenna.)

So when we say "use a balun" we mean "take explicit steps to reduce common mode
current".  That doesn't mean that you have to trade out your center insulator.
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WH7DX
Member

Posts: 1029




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« Reply #27 on: October 06, 2013, 01:05:23 PM »

Quote from: WH7DX

...I was told that I didn't need a balun because the dipoles were balanced...



Do you need a balun?  No.  Many hams use dipoles without them and don't notice
adverse effects that they associate with the lack of a balun.

Is it good practice?  Yes.  It ensures that your antenna works in the way you expect. 
This is true regardless of the number of dipole wires you put on a single feedpoint.

The balun is supposed to prevent common mode current on the feedline.  How much
common mode current there is without a balun depends on a lot of factors, including
the length of the feedline (relative to frequency) and what else is connected to it
in the shack.  Without a balun you may find that the receive noise level is higher,
or the SWR changes when you plug a cable into an unused port on an antenna switch,
or you get RF into your mic circuit.  (In my case my keyer continues sending after
I let off the paddles.)  Most hams don't recognize these as due to lack of a balun, and
sometimes they can be cured by grounding the station or changing the length of the
feedline.

The balun makes your antenna performance more consistent, independent of what
happens to the coax after it leaves the antenna.


Now I should also clarify that in this context "balun" doesn't necessarily mean a
cylindrical object connected between the antenna wires and the coax.
  That is
one physical form, but there are other manifestations.  The ferrite chokes on the
coax, for example, are current baluns because they block (or at least are intended
to reduce) the RF current on the outside of the coax shield - which is what we mean
by "common mode current".  Similarly, ground the shield of the coax 1/4 wavelength
down from the antenna does the same thing (though it doesn't work as well for a
multiband antenna.)

So when we say "use a balun" we mean "take explicit steps to reduce common mode
current".  That doesn't mean that you have to trade out your center insulator.

Thank you!   So the Ferrite Chokes I have in place should be fine with the dipoles.   I'm using LMR-400 coax and it's about 80ft from the QTH to the remote antenna switch.  Then about 50ft to dipoles.   It's about 100ft back to my HexBeam which is more or less above my QTH off the side of the concrete house wall.   I wanted everything to go to the remote switch which is near the property wall etc.   The ferrite chokes are the large ones from Palomor-Engineering (I hope he's ok) with shrink wrap just under the coax / dipole connector. 

The RF I have in my QTH is from my new Samsung 27" monitor.   It only happens during certain times of the day and with certain frequencies.   Usually only 15M and 17M - with 17M the worse.   I don't think I change the monitor settings because it's set to correct size (my preferences etc.).   I tried chokes on the power and video lines but nothing.   It's coming out the screen (housing) I believe.   Not a huge problem and if its at its worst of the day - I turn off the monitor to work a weak signal is necessary - (not on Digital unfortunately).

Thanks for the info!!

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WH7DX
Member

Posts: 1029




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« Reply #28 on: October 06, 2013, 01:08:18 PM »

If you can make your own ladder line you will get slightly better performance than the foam/plastic dielectric commercial stuff. The longest run of ladder line I have is 600 feet long (I love wire antennas).  You do want to keep it off of the ground, I keep it high enough up off the ground so I do not get "clotheslined" while riding on an ATV. Most commercial AM stations still use ladder line and some of their runs are very long (see links to pictures, they look like power lines);

http://gallery.bostonradio.org/2002-03/wsm/100-00497-med.html
http://gallery.bostonradio.org/2002-03/wsm/100-00498-med.html

Tisha

I've seen a video about that place (antenna) and the original broadcasting station.   I can't do that unfortunately.
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N1UK
Member

Posts: 1383




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« Reply #29 on: October 08, 2013, 07:36:56 PM »

Great thread..going back a little and I think that you have rightfully decided against the G5RV but I have to agree that the G5RV is very poor on 15m.  Mine worked well on 80m, 40m and 20m.


I have a coax fed fan dipole with 2 wires one for 40m and one for 30m. It is also resonant on 15m. The dipole is at around 50 feet and works well on 40m for stateside but I find that the 40m vertical works better for further out such as Europe.

Have you thought about putting some sort of remote tuner or antenna switch out at your remote site?  Just throwing ideas out there.


73 Mark N1UK
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