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Author Topic: Lamp wire Norcal Doublet-like antenna?  (Read 1903 times)
KK4RXN
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Posts: 123




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« on: July 18, 2013, 07:29:58 AM »

Researching various antenna constructions I have been interested in the Norcal Doublet-style antenna since it seems so easy to construct. How effective would a lamp-wire (somewhere around 18 gauge) design in this same fashion be?

This may be my Saturday project (as well as my first antenna project) - I have been wanting to make a dipole, but my biggest tree in the front yard fell last night after a big storm. I'm waiting on the dipole construction until I can get all my trees trimmed back so the next one doesn't fall on my house.

Barry
KK4RXN
Jeremiah 29:11-13 / John 3:16
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Barry
KK4RXN
Jeremiah 29:11-13 / John 3:16
KK4RXN
Member

Posts: 123




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« Reply #1 on: July 18, 2013, 07:31:56 AM »

I meant to ask - if it's a decently effective antenna with the lamp-wire, can't I just plug both ends of the wire into my MFJ-949 antenna tuner since it has inputs for wires?

Barry
KK4RXN
Jeremiah 29:11-13 / John 3:16
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Barry
KK4RXN
Jeremiah 29:11-13 / John 3:16
AA4PB
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Posts: 12899




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« Reply #2 on: July 18, 2013, 07:38:53 AM »

Lamp wire should work okay - we used to do that years ago. What's the difference between installing the doublet and installing a dipole? A 1/2 wave dipole for 20M would be 33-feet long and you can directly feed it with coax (RG58 is small). If you trim its length carefully you won't even need a tuner.

The doublet's feed line is balanced so you simply connect one wire to each of the balanced terminals on the tuner.
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WB6BYU
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Posts: 13342




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« Reply #3 on: July 18, 2013, 08:29:16 AM »

I've used zip cord /  lamp wire / speaker wire for dipole feedlines before, and it often
isn't too bad on 40m and 80m.  Losses go up on the higher bands, however.

Let's take an example:  a 40m doublet fed with 35' of zip cord.  On that band the
loss in the feedline is just over 1dB, which is acceptable.  The zip cord has a
characteristic impedance around 100 ohms, so the SWR will be somewhat high at
the rig end (depending on the feedline length) but if you are using a tuner anyway
that shouldn't be a problem.

The same antenna used on 20m now has a feedpoint impedance around 2000 ohms,
and the loss in the zip cord is much higher.  Now only 20% of your power makes it
to the antenna, the rest is dissipated in the wire.  On 15m it is around 60% efficient
due to the lower input impedance of the antenna.

So for a simple dipole on 40m or 80m, zip cord isn't too bad as long as it isn't too
long - say, 50' maximum.  For higher bands, longer lengths, or antennas operated
at high SWR, it probably isn't the best choice.

This is why I eventually switched to RG-174 coax instead of speaker wire for my
backpacking antennas - better impedance match and lower loss (in spite of its
small size.)  But I used tuned dipoles for each band, without a tuner.  300 ohm
twinlead would be better in most cases for a doublet operated on multiple bands.
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KK4RXN
Member

Posts: 123




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« Reply #4 on: July 18, 2013, 02:02:39 PM »

Lamp wire should work okay - we used to do that years ago. What's the difference between installing the doublet and installing a dipole? A 1/2 wave dipole for 20M would be 33-feet long and you can directly feed it with coax (RG58 is small). If you trim its length carefully you won't even need a tuner.

The doublet's feed line is balanced so you simply connect one wire to each of the balanced terminals on the tuner.


The doublet looks like it is so easy to make that I could put it together this weekend, but I'm hoping to put together a fan dipole one of these days soon, too.

Barry
KK4RXN
Jeremiah 29:11-13 / John 3:16
Logged

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Barry
KK4RXN
Jeremiah 29:11-13 / John 3:16
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