WHY LENGTHS ARE IMPORTANT
The Openwire Fed Dipole (OFD) is simply a dipole fed in the middle with any kind of openwire or balanced feedline. It would normally be on half wavelength long on the lowast band you wish to use it on.
As we know, the impedance of the dipole is around 70 Ohms (flat top) or 50 Ohms (Inv.-V) at the feedpoint. If we feed it with 450 Ohms or any other impedance not exactly equal to the feedpoint impedance, there is a transformation of impedance at the shack end of the feedline.
In order to predict the impedance seen at the end of the transmission line in the shack, we must know the following things:
- The Frequency of operation
- The Impedance of the Dipole at the feedpoint
- The Impedance of the Feedline
- The Length of the Feedline
- The Velocity Factor (VF) of the feedline
It is not always easy to know the exact values of of of this so often we can only make a rough estimate. Of course it is possible to exactly measure lengths, impedances and even the VF, but we need an antenna analyzer and a bit of math.
The Potential Problem
Although the impedance on the fundamental frequency is 70 (or 50) Ohms, it is quite different on the upper harmonic bands. The impedance on the 2nd harmonic is about 4000 Ohms. It will vary with height above ground, but it is always very high.
If the length of the feedline coincidentil is one half wavelength (after factoring in the VF), or any multiple thereof (e.g., 1 wl, 1.5 wl, 2 wl, 2.5 wl, etc.) the impedance in the shack will be exactly the same as the impedance at the feedpoint (i.e. about 4K Ohms). This high impedance is impossible for most antenna tuners (matchboxes) to match.
On the other hand, if the length of the feedline happens to be an odd multiple of one quarter wavelength (e.g., 1/4, 3/4, 5/4, etc.) the impedance in the shack may be quite close to 50 Ohms.
If the length of feedline happens to be anywhere inbetween 'a half wavelength (or multiple there of)' and 'an odd multiple of a quarter wavelength', the impedance can be anywhere (all over the map).
Why not simply use a 4:1 Balun?
Mathematically, a 4:1 balun would reduce the 4000 Ohms impedance down to 1000 Ohms, a value nearly every matchbox can match efficiently.
BUT THERE IS A PROBLEM . . .
The little known reality is, a 4:1 balun only performs well when it is closely matched on both ends. In other words, it likes to see 50 Ohms on one side and 200 Ohms on the other side. If the SWR on the antenna side rises, the permance of the balun, specifically, its ability to reject common mode current, deteriorates rapidly.
In general, once the SWR rises above 4:1, the balun is no longer functioning properly. In the case of the center-fed full wave dipole (i.e., 4000 Ohms), this represents an SWR of 20:1. Not good!
It is imperative that we try and find a length of feedline that will transform the impedance down to under 800 Ohms when working with a 4:1 balun.
A BETTER WAY . . . instead of the 4:1 balun, we should ALWAYS USE A "GOOD" 1:1 Guanella balun (not a Maxwell). This has three advantages over using the 4:1 balun:
- It has sufficient Common Mode Impedance (CMI) to assure the Common Mode Current (CMC) is reduced (impeded) enough such that our matchbox does not get hit with CMC.
- Instead of the balun being the limiting factor in the matching range of our setup, the matchbox itself becomes the limiting factor. We are able to use the full matching range of the matchbox. In most matchboxes, this is 1500 to 2000 Ohms.
- In some cases we may actually incur very low impedance at the shack end of the feedline. Often this can be as low as 20 Ohms or less. Using a 4:1 balun in this case would transform the impedance even lower, down to 5 Ohms (or less). VERY BAD! Most matchboxes cannot match this low impedance. The few matchboxes that can actually match it, generally have high internal losses when doing so. THUS THE 4:1 balun is a disaster with low impedances.
Planning The Antenna
If you know what your are doing, have a good enough antenna matchbox for the extremely high impedances, you can use a resonant half wavelength dipole.
If you wish to avoid radical impedances, it is often necessary to choose a more favorable length.
Often you do not have space for a full size (half w.l.) dipole and a shorter version is desirable, but what size shoud you choose.
Over time a few specific lengths have been defined as being favorable.
None are perefect, but some are more favorable than others.
Here is a list of favored Dipole lengths:*
- 40 to 41 meters - 132 to 135 ft.
- 34 to 35 meters - 111 to 115 ft.
- 26 to 27 meters - 85 to 88 ft.
The feedline length is also an important part of a "friendly" antenna design.
When calculating optimum lengths, we must use the velocity factor of the feedline.
This depends on the spacing of the wires of Ladderline (true Openwire).
In general it between 0.95 and 0.98.
When working with commercial Windowline, most people use Wireman feedline.
The Wireman implies that its 450 line is all 450 Ohms (which it is not) and many people often wrongly assume its VF is the same as 300 Ohm TV twinline (which it is not).
The table below (same as on the Info page) shows the portfolio of various models of Wireman balanced line as well as other parameters:
To the best of my knowledge, this table is accurate. Howeveer I have not personally measured this, so buyer beware. Do your own due dilligance.
To be on the safe side, cut feedlines a bit longer and trim as necessary.
TABLES OF RECOMMENDED LENGTHS are shown on the next page.