Linear loaded long-wire

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hertzian

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A simple way to electrically lengthen a short wire is to add a coil inductor to the wire.

Got one laying around? Don't have the parts / time / knowledge to make it a high-Q version? Will you be kicked out of the house with that contraption indoors? :)

Try linear loading. NO, it is NOT as effective as a lumped inductor coil, nor a panacea to all ills. There are plenty of references as to why this is so. Typically you can expect about a .6 to .7 shortening or so as compared to a regular wire, or just a lowering of the resonant frequency.

Basically, linear loading is merely folding a wire back on itself. So instead of using say a 30-foot random wire, try using a 2-wire element folded back on itself.

Example - using speaker wire (or any other 2-wire cable) roll out 30 feet (or whatever you have) Connect the far ends together. At the receiver end, connect only ONE side of the wire to the antenna input jack.

More precisely, you can get in the ballpark with 150 / f Mhz for a quarter-wave element measurement. Dipoles, try 300 / f Mhz. This obviously differs from the standard equations for quarter waves and dipoles since we are now loading the elements.

Nearly all the same rules of physics apply - ie you still need a good ground for verticals, etc, dipoles should be high and in the clear - or just do the best you can.

So if you are short on space, try linear loading your wire and have fun while hitting the books to see where it excels, and where it falls short.

The first tip is that if your wire is extremely long, at some point you'll run into a lot of loss resistance. I typically don't linear load wires longer than about 75 feet from end-to-end. But don't let me stop you from experimenting!
 
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hertzian

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Yikes - maybe I should have put this thread in the antenna forum...

Forgot to mention that a very simple option when running a linear loaded wire like this is to have the ability to just run as a simple single wire again with a switch. That way you can see which works better for you depending on frequency.

At the receiver end, attach both of the antenna leads across the two terminals of an SPST Single-Pole-Single-Throw switch. (At the far end you still have them connected together, no change here) Then run a short jumper from just one of the switch terminals to the receiver.

Now you can switch between a linear loaded wire, or just having two wires connected in parallel basically emulating a single unloaded wire.
 

n5ims

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I used some flat rotor wire (5 or 6 conductor if I remember correctly) to make a multi-band dipole for HF SWL use. I trimmed the individual conductors for the various bands and folded the longest ones back onto the shorter ones to get the desired electrical length while using the minimum physical length. Although it was originally done as a temporary antenna until my tower was raised, it worked well enough that I left it up.

This folding also made it more robust since the multiple conductors provided extra strength. My first attempt didn't fold them back for extra length, but simply removed those conductors so at the end the single conductor didn't last long holding up the full weight of the multiple conductors used for much of the length.
 

ka3jjz

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Yikes - maybe I should have put this thread in the antenna forum...

Forgot to mention that a very simple option when running a linear loaded wire like this is to have the ability to just run as a simple single wire again with a switch. That way you can see which works better for you depending on frequency.

At the receiver end, attach both of the antenna leads across the two terminals of an SPST Single-Pole-Single-Throw switch. (At the far end you still have them connected together, no change here) Then run a short jumper from just one of the switch terminals to the receiver.

Now you can switch between a linear loaded wire, or just having two wires connected in parallel basically emulating a single unloaded wire.
No biggie - it's done....73 Mike
 

hertzian

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More precisely, you can get in the ballpark with 150 / f Mhz for a quarter-wave element measurement. Dipoles, try 300 / f Mhz. This obviously differs from the standard equations for quarter waves and dipoles since we are now loading the elements.
Forgot to mention that with linear-loading, you have a dual-band setup.

The above could be considered the secondary resonance.

For the primary resonance, the linear-loaded wire acts more like just a plain non-loaded type. In this case, we change the values to about 245 / f Mhz for a quarter wave calculation, and 490 / f Mhz for a dipole. I have not seen this exact figure anywhere, it is just what I came up with empirically when testing with the wires attached directly to an MFJ 269 antenna analyzer. YMMV. :)

Example: I just cut a quick linear-loaded vertical out of 11-feet of speaker wire. Shorted at the end, and only connected one side of the wire to the feedpoint.

What I ended up with was a linear-loaded vertical for 13.5 mhz, which also resonated at 22 mhz on the primary.

The easiest thing to do when trying to figure out the two different bands is to initially design for the linear-load to find the length in feet. Then use the primary resonance equation, but apply it to the calculated feet instead of the frequency.

Example: I know that 150/13.5 mhz equals about 11.1 feet for the secondary linear-loaded resonance for my vertical. Where does the primary resonance land?

245 / 11 feet = 22 mhz. (this is for the vertical calculations mind you.)

If you put these in a spreadsheet, it makes it a lot easier to juggle things around to come up with some dual-band configurations to your liking.
 
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