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Zepp antenna advice needed




 
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Author Topic: Zepp antenna advice needed  (Read 11742 times)
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AMLOVER
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« on: September 02, 2012, 06:32:40 AM »

Hi to all,

Moving to a another place I must put a new single band antenna and I have chosen the Zepp as far the new location is so that an endfed is my only solution.
I have attached a schematic of what I can construct at the moment.
L/2 antenna and L/4 owl that ends to a 1:1 linked bal to unbal coupler. The owl can be trimed to any length in order to get 50 ohm in the coupler balanced side.
My first question is, do I need a capacitor connected in the balanced side of the coupler in order to minimize the reactance or the reactance is cancelled as far I can trim the length of the owl to 50 ohm?
In case I need a capacitor, this should be connected in parallel or in series with the balanced side of the coupler for better function?
For the unbalanced side I am thinking to use the inductor link alone as far I'll use the antenna in a single band and only for +- 50khz movements.

All advices are welcome and appreciated.

Stefano


* Zepp.jpg (69.31 KB, 1704x874 - viewed 2434 times.)
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KA0HCP
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« Reply #1 on: September 02, 2012, 08:02:07 AM »

Oh my.  It seems that you are trying to use a "Chinese Menu" to build from, with picking parts from Feedlines, Transformers and Tuners.  Smiley

Here are the approaches:

1. Open Wire Line to balanced tuner
2. OWL to 1:1 or 4:1 BalUn to Coax (short as possible) to tuner
(use whichever ratio best fits with your tuner impedance range.  Some tuners do not like impedances below 50 Ohms).

If this is for use on a fixed frequency, like with a broadcast transmitter, then it would be possible to calculate or measure the precise impedance for that frequency and design a custom BalUn that would yield very close to 50 Ohms, perhaps eliminating the need for a tuner.  However, for best efficiency a tuner should be used.

cheers, bill
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New callsign KA0HCP, ex-KB4QAA.  Relocated to Kansas in April 2019.
W2VW
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« Reply #2 on: September 02, 2012, 11:12:28 AM »

Center feed would be easier to deal with but I suppose that is out for space limitations.

What you have there will work but just use an ugly BalUn with the right amount of turns for the frequency of interest. No need to use coils.

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Bill, KD0HG
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« Reply #3 on: September 02, 2012, 01:46:48 PM »

Use an Inverted L with a tuner at the shack. Don't bother with the open wire feeder.
The Zepp antenna is nothing more then a resonant dipole antenna that is fed at the high voltage point (high impedance end) instead of in the center. This antenna is as old as radio and is probably best known as the Zepp Antenna named after the Zeppelin airship that it was used on.

Rule 1
Get the antenna as high as possible.Both the vertical and horizonal portions.

Rule 2:

Put as best as you can a radial system under this antenna. Because, the RF current flows between the antenna and ground. Lay wires on the ground. Use an adjacent steel fence. Traditional ground rods are worthless.

Remember- Even an infinitely small antenna is only 2 db down from a full sized dipole. As long as you get the losses down with a good ground system.

Bill
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W4NEQ
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« Reply #4 on: September 02, 2012, 03:09:33 PM »

"Remember- Even an infinitely small antenna is only 2 db down from a full sized dipole. As long as you get the losses down with a good ground system."        [ and negligible conductor IR loss ]

Theoretically true, but I'd like to actually see one that efficient .... !



Conversely, a full size ( 1/4 wave) vertical or inverted L without a good radial or counterpoise system is likely to be 3-10 db down from where it can be.  This is the primary reason ham verticals have earned the moniker - "radiates equally poorly in all directions" - few hams install adequate radials / counterpoise.

Only center-fed antennas avoid the need for the "other half."   Conversely, all single-ended antennas require it.  And unless the feedpoint of the single-ended antenna is well away from the shack, you're inviting RF problems unless you run QRP.

Balanced line requires a balanced load if you want it to behave like a transmission line and not a radiator.

Chris
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AMLOVER
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« Reply #5 on: September 04, 2012, 10:15:48 AM »


Zepp is the only way cause of space limitations. No space for a good counterpoise on the ground or over it, no possibility for a center fed dipole as far my place is on the one end of a possible wire antenna.
The only positive in my case is that I can put as much horizontal wire as I wish and so I can fix exactly l/2 antenna + exactly l/4 shorted or owl. In the attachement you can see what my situation is.
I can't use any ferrite balun because of power and I am not sure that an ugly current balun will eliminate the rfi in the shack as far this antenna is not balanced.
I will try the l/4 open wire line with 1:1 air core balun trimmed for 1:1 swr as in my previous schematic and I'll see what devils I'll get in the shack and in the air.
Thank you all.

Stefano


* Zepp 2.jpg (55.9 KB, 1704x874 - viewed 1172 times.)
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« Reply #6 on: September 04, 2012, 12:02:05 PM »

Maybe you can install the balanced line in line with the axis of the rest of the antenna. That may help with rf in the shack since the balanced line acts as counterpoise for the end fed section. 
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N8LGU
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« Reply #7 on: September 06, 2012, 11:09:23 AM »

Try dropping a 68' peice of black hookup wire from the unused side of the OWL down the side of the apt building. Use a lead sinker to hold it down. That way you will have a counterpoise.
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"Rock Cave Dave"
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« Reply #8 on: September 06, 2012, 12:23:31 PM »

What is the power output of the transmitter?  This is the perfect situation for the MMD miracle antenna.  The MMD is low power only though. It is a standard dipole but end fed.  They work very well for what they are. 

C
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AMLOVER
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« Reply #9 on: September 06, 2012, 02:23:28 PM »


I simulated at "mmana" a single wire from the open end of the owl to some possible directions but that distorted the lobe very much. The only positive direction was in the same axis to antenna but opposite way like a double zepp. If I leave one leg on the air then I'll have infinite resistance there and about 6-9 Kohm on the other side. If I'll use the 68" counterpoise then possibly I'll get few Ohms there and 6-9 Kohms on the other side.
What condition will be more balanced? I don't know.
I must live from now on with an end fed antenna so I have to fight with its unbalance and hopefully to win it...
It sounds definately interesting to short the l/4 owl and use the total 3/4l antenna as a horizontal J, to feed it some percentage from the shorted end where is the sweet 50 ohm spot and add an adequate ugly balun for rfi.
Any other suggestion?
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W4NEQ
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« Reply #10 on: September 06, 2012, 03:44:53 PM »

Judging from your diagram, I would locate the best building ground and work the single wire against that.  Not much point in open wire line without a balanced load.

Electrical codes notwithstanding, most multistory buildings have lightning rods around the roof perimeter interconnected with very heavy cable.  That or perhaps a bolted connection to some structural steel would make the building structure the counterpoise, probably the best you're going to get.  Place an LDG autotuner at the remote feedpoint, and run coax back to the rig ... 

For single band use, a quarter wave is easiest to match, For 80/40, I'd try 90 feet, for 80/160 I'd use 180 ft.  You might have to see what the tuner likes.

I do something similar with a vertical on my boat with a large piece of sheet aluminum thrown overboard.

Chris
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W8EJO
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« Reply #11 on: September 07, 2012, 08:18:45 AM »

For 80-10, use an 87' end fed wire with counterpoise(s) of about .05wl. Feed via link coupled tuner (matchbox).

Works good, loads up on all bands - no RF.

I use this while camping. I find the counterpoise length not to be critical.

This is based on the work of G3CCB in "Taming the End-Fed Antenna".  See RGSB's "The Antenna File".

Other lengths will work depending on bands of interest.


 
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Terry, W8EJO

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