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Author Topic: 50 foot doublet feeder length  (Read 2103 times)
M0JHA
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Posts: 646




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« on: April 24, 2013, 06:29:09 AM »

i wanted to make a 50 foot doublet for 40/30 and 20 using the internal tuner on the KX1 . Can anyone suggest a length of feeder to keep matching needs to a minimum ( 300 ohm unslotted ribbon feeder) ?

I was going to make the Norcal doublet which is 22 foot per side and 28 foot feeder but it doesn't use 300 ohm feeder and i'm not sure if that would need an 4.1 tranny if used with the KX1 ..

any advice appreciated

billy, m0jha
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GW3OQK
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Posts: 147




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« Reply #1 on: April 24, 2013, 09:13:39 AM »

I would use any old feeder length plus a home wound 1:1 balun and give it a try.
Andrew
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M0JHA
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Posts: 646




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« Reply #2 on: April 24, 2013, 09:44:15 AM »

Yes , i need 25 foot to get from feedpoint to where radio would be so may start with slightly longer in case i need to trim , i'm just being tight as i have a 20m roll of feeder and don't want to go wasting too much chopping away at it if i can help it ..
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GW3OQK
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Posts: 147




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« Reply #3 on: April 25, 2013, 11:12:44 AM »

Billy, the tx spec claims it will tune practically any random wire so I then expect it to tune a random length dipole via a random length twin lead plus a balun at the tx and not know the difference. Let us know how it works out please. What's great is to experiment. If you want a sked to prove it let me know.

I have used the Z-match at the end of 300 & 450 ohm ladderline and the aerial length and feeder length never seems to matter. The Z-match just tunes the system to a 1:1 swr (on the coax to the radio) and radiates well.

Andrew
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WB6BYU
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Posts: 13280




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« Reply #4 on: April 25, 2013, 12:03:22 PM »

My only concern is that the antenna is close to resonant on 30m, and a quarter wave of
line (about 20 feet or so) may step the impedance up to an inconvenient value.  A quick
model suggests the SWR on 300 ohm line is only 4 : 1, so the maximum impedance you
would encounter is 1200 ohms.  That's probably tolerable.

Here is one way you can check the values yourself:

W9CF has a handy yagi modelling applet you can run online here:
http://fermi.la.asu.edu/w9cf/yagipub/index.html

Create a single element yagi with a 50' element (using perhaps 25 segments for good
measure) and see what feedpoint impedance it gives you for each band.

Then you can stick the impedance into VK1OD's transmission line loss calculator here:
http://vk1od.net/calc/tl/tllc.php

for various lengths of 300 ohm line and see what the resulting impedance is at the tuner.
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M0JHA
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Posts: 646




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« Reply #5 on: April 26, 2013, 02:39:03 AM »

Thanks for the replies chaps , i have just made a 55 foot doublet with 25 foot of 300 ohm feeder to a 1.1 balun and then a very short piece of 174 ( about 7 inches) I was talking to steve G3TXQ who suggested this config and modelled it, i  would need to strap a 180pf cap across the feeder for 40m which isn't a hassle as the balun/fedder will be near the radio ..

So if wx today is fine i will string it up in the back and see how i get on ..

i'm ordering 2 winders that are used with the yo yo antenna and will make one of those i think .

The KX1 doesn't have a great matching range , 4-4uh and 140pf so the cap strapped across the feeder will just hel;p the atu out a bit ..

i will probably end up just playing with "random" wires  to be honest , i don't want to get to bogged down when my original idea of having the KX1 was simplicity and ease of just chucking a wire out and have fun ..

it's all fun messing anyway
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