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Author Topic: Increasing efficiency of shortened dipole  (Read 3877 times)
KJ6QLD
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Posts: 22




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« on: February 15, 2012, 11:10:57 PM »

So far I've learned from my reading about shortened dipoles that you typically lose efficiency in two primary ways:
1 - you add coil losses (if using a coil) and
2 - as the physical length decreases, so does the radiation resistance (whether your use hats/coils/both). 

Putting #1 aside for now, and with #2 in mind, I've also been reading a bit about off center fed dipoles, and have learned that the further away from the center you feed them, the higher the radiation resistance. 

So this leads me to my question:  Could the efficiency of a shortened dipole be increased by feeding it off center?  Could the loss in radiation resistance due to physically shortening it be then made up for by feeding it off center, (and thus mitigate reason #2)?

Thanks,
Galen
KJ6QLD
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G3TXQ
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Posts: 1464




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« Reply #1 on: February 16, 2012, 01:08:32 AM »

It would do nothing to reduce the losses in the dipole itself, because essentially the same current distribution must exist along the dipole even when you feed it off-centre where the current is lower. There may be some benefit because of reduced current in the feedline/ matching network.

We have to be very careful when calculating antenna efficiency that we understand what definition of "radiation resistance" we are using, and that we are referring all resistances - radiation resistance and loss resistances - to a common point. Tom W8JI has a useful page on the topic:

http://www.w8ji.com/radiation_resistance.htm

73,
Steve G3TXQ
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KJ6QLD
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Posts: 22




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« Reply #2 on: February 16, 2012, 08:39:23 AM »

Good link - thanks
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WB6BYU
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Posts: 13028




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« Reply #3 on: February 16, 2012, 09:05:15 AM »

As explained above, the answer is NO.

The radiation resistance - that is, the total current that has to flow in the antenna to radiate a
fixed amount of power, is based on the total length (and the loading method), not where you
feed the antenna.

For example, using a folded dipole (two wires in parallel) increases the input impedance by a
factor of 4, but the radiation resistance remains the same because the same total
current is spread between the two wires.

There may be times when feeding a short antenna off-center makes it easier to match because
the feedpoint impedance is higher, but that isn't the same as raising the radiation resistance.
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WX7G
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Posts: 5920




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« Reply #4 on: February 16, 2012, 09:09:12 AM »

Feeding off of center raises the FEEDPOINT resistance but does not change the RADIATION resistance. The current distribution along the dipole is the same for center feed and offset feed.

To raise the radiation resistance of a short dipole the "current-area" needs to be increased. This is the current integrated along the length. For example, say we have a 28 MHz dipole that is 1 meter long and it does not have end loading structures (top hats). And we feed it with 1 amp at the center. The current tapers linearly from 1 amp at the center to 0 amps at the ends. The average current along the 1 meter length is 1/2 amp. The current-area is 1/2 amp X 1 meter = 1/2 A-m.

The radiation resistance is proportional to the square of the current-area. A full half wave dipole for 28 MHz has a current area of approximately 3.5 A-m. Note that for a full half wave dipole the shape of the current along the length is close to a half sine.

What is the approximate radiation resistance of our 1 meter dipole? [(0.5 A-m)/(3.5 A-m)]^2 X 72 ohms = 1.5 ohms.

Now, let's say we add end loading structures to our 1 meter dipole. The current distribution is almost constant. It is 1 amp at the center and 1 amp at the ends. The current-area is not 1 amp X 1 meter = 1 A-m.

What is the approximate radiation resistance of our end-loaded 1 meter dipole? [(1 A-m)/(3.5 A-m)]^2 X 72 ohms = 6 ohms.

End loading has quadrupled the radiation resistance of the antenna. This is the most effective way to increase the radiation resistance of a short dipole or monopole.

The end loading structure decreases the amount of loading inductance needed. Neglecting loading coil losses the Q of the antenna is X/R where X is loading coil reactance and R is radiation resistance. Let's take the case where end loading has cut the loading coil reactance by a factor of 2 and increased the radiation resistance by a factor of 4. The Q has dropped by a factor of eight and the bandwidth of the antenna is 8 times greater.

Dave's rule of short antennas (that's me):

SHORT
EFFICIENT
WIDEBAND

PICK ANY TWO
« Last Edit: February 16, 2012, 09:11:50 AM by WX7G » Logged
KJ6QLD
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Posts: 22




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« Reply #5 on: February 16, 2012, 01:05:20 PM »

All responses helpful and on the money.  As was suggested, I had confused feedpoint resistance with radiation resistance.  I think I have enough conceptual understanding now to start experimenting with a shortened dipole.

While we're on the topic, a further question:  Given my intended mounting location, I need to have at least the upper half of the vertical dipole shortened.  I've got plenty of room, though, to use a full length 1/4 wave radiator for the lower half.  I conclude that it would be more efficient to go ahead and use the full 1/4 wave wire for the lower half (as well as a lot cheaper and easier).  The antenna will, though, be obviously very asymmetrical physically speaking, with a shortened, loaded upper radiator and a straight full length lower radiator.  Are there any drawbacks or pitfalls that I should be aware of with this configuration?
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N4JTE
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Posts: 1154




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« Reply #6 on: February 16, 2012, 02:22:42 PM »

Just let the ends hang down from a non conductive support, keep dipole full size.You can shorten the point to point size by 25% without a noticable decrease in efficency.
Bob
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WB2WIK
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« Reply #7 on: February 16, 2012, 03:16:53 PM »

Place shortened dipole between the tops of two 200' tall towers and just use it, it will work okay. Smiley
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W5DXP
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Posts: 3546


WWW

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« Reply #8 on: February 17, 2012, 07:09:08 AM »

I think I have enough conceptual understanding now to start experimenting with a shortened dipole.

The same techniques used to increase the efficiency of mobile antennas can be used to increase the efficiency of shortened dipoles, center-loading with high-Q coils, top (end) hats, etc.
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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.
WX7G
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Posts: 5920




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« Reply #9 on: February 17, 2012, 07:19:01 AM »

All responses helpful and on the money.  As was suggested, I had confused feedpoint resistance with radiation resistance.  I think I have enough conceptual understanding now to start experimenting with a shortened dipole.

While we're on the topic, a further question:  Given my intended mounting location, I need to have at least the upper half of the vertical dipole shortened.  I've got plenty of room, though, to use a full length 1/4 wave radiator for the lower half.  I conclude that it would be more efficient to go ahead and use the full 1/4 wave wire for the lower half (as well as a lot cheaper and easier).  The antenna will, though, be obviously very asymmetrical physically speaking, with a shortened, loaded upper radiator and a straight full length lower radiator.  Are there any drawbacks or pitfalls that I should be aware of with this configuration?


The trick is bringing the feedline away from the vertical dipole. Are you bringing it away at a right angle from the feedpoint?
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KJ6QLD
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Posts: 22




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« Reply #10 on: February 17, 2012, 08:20:27 AM »

Yes, I will be bringing the feedline off at pretty close to a right angle.  I'll also be using a current balun at the feed point.
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