20 Meter Antenna

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I have a twenty meter dipole and according to the box it is 34 feet long.  I am going to be feeding it with coax due to interence problems from devices in our house and believe that twin lead feedline would allow interference.  Would it be possible to use it on ten meters as a full wave dipole and forty meters as quarter wave dipole without a transmatch or using twin lead or ladder line?

Pete Allen:
Hi: 34 feet is about right for a 20 Meter dipole. Depending on your antenna's height above ground feedpoint impedance will be between 20 and 120 ohms at resonance. And it should be just about right for the CW portion of 20 as is. So your SWR will be well within reason. In fact, if you are at all lucky and you hang it at the right height, you will have a perfect 1:1 match to quality coax.

Now, using any dipole on another band is a problem. The feedpoint impedance gets completely out of hand. On 40, the feedpoint impedance will be around 15 ohms with about 1,000 ohms of capacitive reactance. So your SWR will be 1000:1, more or less. No tuner is going to be able to match that!

The other way, using a full length 20 M dipole on 10, the feedpoint impedance will be very high. How high depends on a lot of things, but the SWR will be somewhere between 50:1 and 250:1. And no tuner is going to match that either.

The SWR is the ratio between forward and reflected power. If you have enough feedline, and if your feedline is lossy enough, a tuner will give you an apparent match to the feedline. You aren't "tuning the antenna," you are only matching the transmitter to the feedline. So virtually all the power you put into such a mismatched system will be converted to heat by your transmission line losses. Not good.

Use your 20 M dipole as it was intended to be used. On 20 Meters. When you want more bands, get a multi-band antenna that will match your feedline, or put up multiple antennas.

Oh yes - use quality coax. RG213 or one with even lower losses.

73  Pete Allen  AC5E

I recently set up a 10/20 meter wire dipole with a single feed line that just by chance, I was also able to transmit and receive 15 meters on. Make sure to check your SWR. BTW, I added a correct third wire specifically for 15, just to make things right. It only cost about $5 for wire and insulators.

John Nogan:
An ideal feed point impedance of a 1-wavelength "resonant" dipole is about 90 ohms resistive (fed at the center or current loop). This should not be a problem for your radio's internal antenna tuner. Feed point resistance at 10-wavelengths only climbs to about 160 ohms, resistive.

As far as 1/4-wave operation, feed point impedance will be in the neighborhood of 7-j1000 ohms (or 15-j1000). This impedance should not be a problem for a good quality "T"-type transmatch with roller inductor. I personally ran a 1/4-wave dipole on 160m for years with decent results. Efficiency of a 1/4-wave dipole is about 90-95% so it should work well for you.

As far as ladder line, if applied correctly it should not radiate. Currents in each leg of a balanced feeder (ladder line) are 180 degrees out of phase and therefore cancel - no radiation. If an imbalance occurs in the ladder line due to improper application (real world), the currents are no longer 180 degrees out of phase and feed line radiation will occur. Whether or not the amount of radiation is enough to cause you a problem is questionable.

On a final note, if you use coax cable use RG-8 or RG-213 (voltage rating for off resonant operation). Also, place a balun (current type) at the feed point of the antenna to minimize the potential for feed line radiation.

John Nogan:
Correction, 1-wavelength antenna center fed is not fed at a current loop is  therefore the impedance at the feed point would be high.  Sorry for the confusion. Your radios antenna tuner will not be happy with this combination.  Regardless, a good "T"-type transmatch as described above should tune the antenna with little difficulty.


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