Extended Double-Zepp
(Psst...Wanna double your power, for Free?)
Well, not for free, but almost so. Let's look at how one can "double" one's power. How, first of all, are we looking at your "power". One way is to look at the output of your rig. If you increased your output from 5 watts to 10 watts, there could be no argument that you had doubled your power. But, if your antenna had a gain of 2 [3dB], at the other end of your contact, it would LOOK like you had increased your power.
Now, about the "FREE" part of this discussion. The simplest and least expensive antennas are wire antennas. To double your Effective Radiated Power [ERP], you need a gain of 3dB over your former antenna. Using a dipole as the reference, this would be expressed as 3 dBd, or 3 dB over a dipole. So, what "FREE" wire antenna will give us 3dBd?
One of the easiest and cheapest is the Extended Double-Zepp, or EDZ. It is two elements, fed in a collinear fashion, with each element extended from 1/4-wave long to 5/8-wave long. Many of the Antenna Books show this antenna fed via a 0.11-wavelength stub, which results in a 150-ohm feedpoint impedance. Nice, but when was the last time you saw 150-ohm feedline? So, you say, 'How do I feed my EDZ?' If you feed it directly with ladder line, either 300-ohm, or 450-ohm, the SWR on the line will be 2:1 or 3:1. With the inherent low loss of the ladder lines, this will never be noticed. The ATU you are using will tune out this mismatch, and make the transmitter happy. If you really insist on a 'flat' line for the antenna, then a 1/4-wave shorted line should be attached to the feedpoint, and your feedline should be tapped down to the correct spot on the line to match it. Using this method, you could even feed the EDZ with 50-ohm line. This would make the antenna a single-band antler, whereas the resonant feed with the ladder line will allow you to run the antler on multiple bands.
The formulas used to calculate the lengths in an Extended Double Zepp are as follows:
Leg length (feet): L= 600/F(MHz)
Stub length (feet): l08/F(MHz)
[-------------L---------------] [---------------L-------------]
------------------------------o[ins]o------------------------------
\ / :
! ! :
! ! :
! ! [ H ]
! ! :
! ! :
o o -----
Feedpoint
***NOTE: [ins] is the center insulator for the wire.
If you want your EDZ to be the single band version, and not use an ATU, then the stub length, [ l ], should be equal to 246/F(MHz), and the bottom of the stub should be connected together, with the feedline tapped up to the correct matching point. This point would need to be determined empirically, also known as "cut-and-try".
|
FREQ (MHz) |
L (Ft) |
H (Ft) |
Flattop Length (Ft) |
|
1.85 MHz |
324.3' |
58.4' |
648.6' |
|
1.95 MHz |
315.8' |
56.8' |
631.6' |
|
3.60 MHz |
166.7' |
30.0' |
333.3'* |
|
3.90 MHz |
153.8' |
27.7' |
307.7' |
|
7.15 MHz |
83.9' |
15.1' |
167.8'* |
|
10.10 MHz |
59.4' |
10.7' |
118.8' |
|
14.20 MHz |
42.3' |
7.6' |
84.5' |
|
18.10 MHz |
33.1' |
6.0' |
66.3' |
|
21.20 MHz |
28.3' |
5.1' |
56.6'* |
|
24.90 MHz |
24.1' |
4.3' |
48.2' |
|
28.40 MHz |
21.1' |
3.8' |
42.2'* |
|
29.50 MHz |
20.3' |
3.7' |
40.7' |
|
50.20 MHz |
12.0' |
2.1' |
23.9' |
|
52.50 MHz |
11.4' |
2.1' |
22.9' |
|
* - Very Good Novice Antenna(s), also |
|||
HOW BIG IS IT?
This is a good question, and might be the disqualifying factor for some. Here is a listing, in tabular form with dimensions. I am listing it for the bands from 160 meters up, but I don't have the room for the lower bands, myself. [Wish I did, though].
As you can see from the chart, many of the dimensions for an EDZ are the same as a dipole for other bands. This helps explain why a center-fed antenna, fed with ladder line, will work so well on multiple bands. It is a 1/2-wave on one band, 1-wave on others, and EDZ on yet others. Another principle that allows one to "get away" with the ladder line feed system is the good old "Gootches' Principle", which states that "RF Gotta go SOMEWHERE!"
So, if you have the real estate, the time and the inclination, try out an EDZ in your Antenna Farm, or for Field Day, and see what sprouts up in the bands, and in your log...