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Author Topic: offset tap on a single wire antenna - why?  (Read 5626 times)
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Patrick J. / KD5OEI
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« on: August 06, 2011, 05:02:54 PM »

This has been puzzling and my question is, do I understand it corectly?

Here is my explanation and the attachments show my reasoning based on what I read:

1.) By offsetting the feed point, one gets two antennas: one that resonates at a lower frequency and one at a higher. The result is a setup that can cover the amateur band in question.

2.) The lengths resulting from the offset to the feed point create resonances far outside the ham band, yet the total length is right on the specified frequency. The combination of these factors creates an antenna that strongly resonates at the specified frequency but has a bandwidth that covers the ham band without the fixed match of the transmitter becoming usable and this avoids having to change the antenna coil tap position.

A general question that might follow this line of reasoning is: What if I changed a ladder-line-fed dipole so that its 'center' tap was offset in this same manner?


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Radio Candelstein - Flagship Station of the NRK Radio Network.
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« Reply #1 on: August 06, 2011, 05:15:11 PM »

That's what makes this board great:  Authentic western radio gibberish!  Smiley

Is this from an old manual?

I'll have to leave this to someone else to address as a whole.

bill.
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kb3ouk
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« Reply #2 on: August 06, 2011, 07:07:31 PM »

that looks to me like that is supposed to be a windom antenna.
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Ed/KB1HYS
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« Reply #3 on: August 06, 2011, 07:34:06 PM »

offcenter feed points are also supposed to provide a feedpoint impedance that may provide a better match feedline to dipole.  A 72 ohm impedance is only found at th center of a half wave wire. Sliding the feepoint to another point will change the impedance seen by the feedline. 

I honestly can't speak to the resonant effects, but that does make sense to me if you split an antenna into different 1/4 wave parts they should work. Like a fan dipole with only two parts.
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73 de Ed/KB1HYS
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kb3ouk
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« Reply #4 on: August 06, 2011, 07:35:25 PM »

but the picture only shows a single feeder, just like a windom.

actually, the antenna described in the article is exactly what my antenna book describes as being a windom antenna. a wire antenna fed by a single wire connected at approx. 14% off-center. the single feeder is off centered to provide a match to the antenna for the feeder wire. as to why someone would want an antenna like this, my guess is that because it is fed like a randomwire, but a major chunk of the radiator is still outside of the shack.
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« Reply #5 on: August 06, 2011, 10:36:38 PM »

The material is from the Hallicrafters HT-9 manual in the section on the antenna. The manual makes no mention of impedance the transmitter is supposed to see but does talk about moving the tap on the output coil. It describes setting up this kind of antenna in much more detail than any other kind so it must have been what worked for that transmitter. One terminal to the ant wire, one to ground.

Just want to understand the ant. I can see it being somewhat like half a fan dipole. I think it can be assumed that the wire going to the antenna tap spends very few feet indoors, then runs in an arc up to hang from the antenna's horizontal wire at the correct spot. I guess that wire would certainly radiate too.
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« Reply #6 on: August 06, 2011, 11:12:04 PM »

That sort of unbalanced feed is asking for a lot of soup in the shack.

The good thing is a few loose NE-2 lamps laying around will let you know you are on the air.

73DG
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VE3DDY
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« Reply #7 on: August 07, 2011, 09:31:33 AM »

This has been puzzling and my question is, do I understand it corectly?

Here is my explanation and the attachments show my reasoning based on what I read:

1.) By offsetting the feed point, one gets two antennas: one that resonates at a lower frequency and one at a higher. The result is a setup that can cover the amateur band in question.
---------------------------------------------------------------------------------------------------------------
OK. By feeding the antenna at the 1/3 rd point the impedance is at 300 ohms on several bands.
If you plot the Current curves for 80, 40, 20 etc  on a 132 ft length you would find that several of them cross at the 1/3 length position. At this point the Impedance is approximately 300 ohms. From this point run 300 ohm twin lead down to the rig.
So then you require a 4 to 1 balun at the bottom end if you are using 75 ohm coax and a 6:1 if you are using 50 ohm coax.
You can put the balun right up at the division in the antenna and run the appropiate coax to the rig from there Otherwise run 300 ohm twin lead and put the balun at the rig.
Jim VE3DDY
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WD5JKO
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« Reply #8 on: August 07, 2011, 09:50:23 AM »



Could be that the HT9 loads well into 300-500 ohm loads, and an old fashion single wire fed Windom fits that bill nicely. Since the Windom will radiate all harmonics well, that needs to be a consideration if you do this today.

My antenna here is a big compromise Windom. I use a 38' insulated push up mast, and a 130' offcenter fed Windom attached to the tower top. Interestingly being low to the ground, and in an inverted V configuration, I see < 2:1 resonant points on 160m, 80m, and just above 40m, and 20m. It takes little effort to tune this for 160-15m operation. Operation is poor on 160, but makes good signals on 80-20m.

I currently feed it with 12' RG8X with the coax wrapped many turns through a 2" OD ferrite core at the antenna feedpoint to act as a current balun against a few radials and a ground rod. I am using a Johnson 275 matchbox at the rig. This works for 20m, and 80m. For 40m, I must tune at the antenna end. RF in the shack is not an issue with the current balun.

Jim
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