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Author Topic: Feeding a trapped dipole with 450u ladder line?  (Read 2259 times)
KT0DD
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Posts: 278




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« on: January 26, 2013, 08:28:04 AM »

Is feeding a trapped dipole with 450 ladder line a good idea? I'm thinking that most trapped dipoles are based on a 50 ohm feedpoint, and also matching the length of the 450 feedline to be acceptable on multiple bands with traps in the antenna would pose a problem?

Any info is appreciated.

73, Todd - KT0DD
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WB6BYU
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Posts: 13335




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« Reply #1 on: January 26, 2013, 09:12:39 AM »

It will work.  How well depends on your operating needs, length of feedline,
whether you are planning to use it on bands that don't have traps, what
radiation pattern you want, etc.


If you are just doing this to reduce feedline losses, it might or might not
actually do that. You can use VK1OD's transmission line loss calculator:

http://vk1od.net/calc/tl/tllc.php

to compare the relative losses with Mismatch:  ZLoad = 50 ohms.

For example, I compared 100' of RG-213 with Wireman 551 feeding a
50 ohm resistance on 20m and the ladder line was very slightly higher
loss.  That would probably argue that, given the need for a tuner to
match the ladder line, the coax feed probably would be a better choice,
at least on the bands where the dipole is resonant.
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AA4PB
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Posts: 12892




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« Reply #2 on: January 26, 2013, 09:20:18 AM »

What is it that you hope to gain with the ladder line? You aren't going to be able to force the trapped dipole to function on bands it was not designed for. The trapped dipole will have a feed impedance of 50-75 ohms so it won't be a good match for 450 ohm ladder line.

Feeding a doublet (at least 1/2 wavelength on the lowest frequency and no traps) with ladder line can allow you to tune the system on multiple bands - but you can't do that with traps in line.
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W5DXP
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« Reply #3 on: January 26, 2013, 10:09:14 AM »

Is feeding a trapped dipole with 450 ladder line a good idea?

One of the reasons for having a trapped dipole is to avoid an antenna tuner. When you replace the coax feedline with ladder-line, you are guaranteeing the need for a tuner. If the antenna has a 50 ohm feedpoint impedance and you happen to feed it with an odd multiple of 1/4WL of ladder-line, the impedance at the tuner will be around 4000 ohms. You might as well just have a single-wire dipole with no traps because the impedance at the tuner rarely gets any higher than 4000 ohms.
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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.
AA5WG
Member

Posts: 498




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« Reply #4 on: January 26, 2013, 12:57:34 PM »

Hi Todd,

Many feed their balanced antenna with ladder line, home brew ladder line is best, along with a balanced link antenna coupler for multi-band use.

Chuck
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KT0DD
Member

Posts: 278




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« Reply #5 on: January 26, 2013, 04:16:31 PM »

I was thinking for longer runs, ladder line might be less lossy than coax. I plan to have a tuner anyway in case I want to operate on segments of the bands that might be too far away from the resonant point of the antenna on 75 / 40m. I usually find I have good bandwidth on the higher bands from 20m up, and may not need the tuner there in which case I will bypass the tuner.

Thanks for the info. I will try VK1OD's modeling program.

73, Todd - KT0DD
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WB6BYU
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Posts: 13335




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« Reply #6 on: January 26, 2013, 09:00:49 PM »

You'll get better bandwidth (that is, amount you can change frequency without
needing to readjust the tuner) when the feedline is operated at a lower SWR.
This is especially important as the line gets longer.

For example, if you are using 35' of 600 ohm line to feed a 50 ohm load on 20m,
(1/2 wave, so the impedance repeats) the SWR relative to 50 ohms is 1.15 : 1
at 14.0 MHz and 1.9 : 1 at 14.3 MHz.  If I lengthen the line to 210.7 feet (6
half wavelengths) the SWR is 1.2 at 14.0 MHz but almost 20 : 1 at 14.3 MHz.


We've had discussions about using open wire line to reduce the losses in long
feedlines before here in eHam, and the general consensus seems to be that
you are better off looking for the end of a spool of 75 ohm CATV trunk coax
or other low loss coax. 

Personally I'd probably try building a 4-wire 200 ohm open wire line ("Type XVI"
in Laporte's Radio Antenna Engineering) and match it with 4 : 1 baluns at
each end, but I'm weird.
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N4CR
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Posts: 1672




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« Reply #7 on: January 27, 2013, 07:36:26 AM »

Personally I'd probably try building a 4-wire 200 ohm open wire line ("Type XVI"
in Laporte's Radio Antenna Engineering) and match it with 4 : 1 baluns at
each end, but I'm weird.

I'd probably use a tapered matching section on both ends.
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73 de N4CR, Phil

We are Coulomb of Borg. Resistance is futile. Voltage, on the other hand, has potential.
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