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Author Topic: Feed 1/2 wave dipole with 300 ohm line?  (Read 358 times)
VA7RWR
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Posts: 30




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« on: September 08, 2006, 03:40:51 PM »

Reason I'm asking is that the window I'll be running the feed line down to is a push out style, not the traditional window pane where you slide the window up to open it.  So I can't pull the window shut without severely pinching 50 ohm coax.  I can close it on the flat TV line style of feed line, so with my MFJ 941 tuner could I get it to tune with 300 ohm flat line?
I need ot be able to pull this window shut tight and lock it as it'd be too easy to get in to our suite with it open.

thx
Ron
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KC8VWM
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Posts: 3119




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« Reply #1 on: September 08, 2006, 03:48:12 PM »

Someone recently told me that using "window alarm foil tape" can be very handy for making an antenna feedline conduit under the window frame for this purpose.  

73
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W5DXP
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« Reply #2 on: September 08, 2006, 04:58:40 PM »

If you feed it with N*1/2 wavelength of 300 ohm line, you will probably not notice the difference - and you will have a multi-band antenna.
--
73, Cecil, w5dxp.com
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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.
N3BIF
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Posts: 1190




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« Reply #3 on: September 08, 2006, 06:47:37 PM »

 Yes without reservation!   Go, and do.
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N4CR
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Posts: 1666




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« Reply #4 on: September 09, 2006, 12:51:39 AM »

If the window frame that you are pinching onto the wire is metal, it will effect the twin lead adversely. How much depends the bulk of the metal that is very near the twin lead. Even small amounts of metal near twin lead will cause loss or impedence humps.
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73 de N4CR, Phil

We are Coulomb of Borg. Resistance is futile. Voltage, on the other hand, has potential.
VA7RWR
Member

Posts: 30




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« Reply #5 on: September 09, 2006, 09:34:23 AM »

Ya, there's no way of getting around the fact that it's a metal window frame.  I guess I'll just have to open the window when TXing so that the cable can hang free away from the frame.
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N2IK
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Posts: 220




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« Reply #6 on: September 09, 2006, 05:52:51 PM »

Perhaps you could replace the glass in the window with acrylic or polycarbonate plastic and drill for feedthrough insulators for the balanced line and whatever you plan for the future.

73 de Walt N2IK
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KB4QAA
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Posts: 2371




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« Reply #7 on: September 10, 2006, 03:45:19 PM »

1.  Give it a try.  If it works good enough for you, it's fine.  I use TV twin lead while traveling with my qrp rig and a dipole and I can get out.

2.  In making a window pass thru, I always make mine out of wood.  I either drill holes with vertical notches, or you can stack two peices on top of one another.  Paint them to match the window frame, and put a piece of dowel from the movable sash to the top of the frame to keep burglars from raising it.  Stuff the gap between the windows with foam to keep the cold out.

good luck,  bill
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W5DXP
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« Reply #8 on: September 11, 2006, 05:45:45 AM »

> KB4QAA wrote: Stuff the gap between the windows with foam to keep the cold out. <

Actually, you are stuffing the gap to keep the heat in. :-) Here in Texas, we stuff the gap to keep the heat out.
--
73, Cecil  w5dxp.com
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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.
VA7CPC
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Posts: 2386




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« Reply #9 on: September 11, 2006, 06:30:28 PM »

Is it possible to use capacitive coupling to connect the inside and outside of the transmission line?

Terminate each wire (inside and outside) on a metal pad cemented to the glass, and let the capacitance of the pads (from inside pad to outside pad) carry the signal?

[Without texts, I can't estimate what the reactance of a 1"-square, window-glass-thick capacitor would be.  Maybe too small to be useful. or we'd all do it like that!]
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