OCF Windom antenna performance
I have been testing a OCF antenna version with 12,96' ( short leg ) 52,89' ( long leg ) plus 10' vertical radiator with the coax feed line where I attached an 1:1 chocke balun.
Regarding to the RF on shack I'm not seen troubles since I have instaled the 4:1 current baliun and 1:1 chocke ( composed by 8 turns of RG58 coax over a TV flyback nucleos ferrite ).
This antenna gave me 40m ( 1:3 ) 20m ( 1:4 ) 15m ( 1:3 ) 10m ( 1:1 ), the main question is related to the gain, I am wondering that for 40m it should be the same as a 1/2 wave dipole but how about the rest of the operational bands ? Does anyone know it ?
Please give your commets.
Last edited by PY2CAA; 01-16-2012 at 04:36 PM.
Without traps to section the antenna into halfwave lengths on each band the directionality will be broadside for lower bands and end fire on the higher bands just like any long dipole.
Does not matter if you feed from end or in middle , that just affects the feedpoint Z !, not gain.
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Ok, the main changes are related to the Z but does the fact to have an antenna cut to 1/2 wave of 40m will not affect the gain when it is used on 20, 15 and 10m as well ?
Should I have better performance on these band using the OCFD (1/2 on 40m) then a single 1/2 wave for each specific band ?
If the 4 to 1 balun is a good current mode balun, the 10 ft section should not create any significant radiation on any band, and the antenna should function like an OCF. However a regular OCF of those dimensions should give much lower SWR than you are measuring. So either there is a problem in stating the SWR or there is something else happening with the antenna to cause the high SWR.
Do you mean SWR (1:3) the same as 3 to 1 SWR?
The choke wound on a TV flyback transformer core will be primarily an inductance since those are low loss cores. That can increase or decrease the common mode currents depending on feedline length, antenna dimensions, and a several other parameters.
A normal OCF of those dimensions (assuming no feedline radiation) will have a half wave dipole pattern on 40 meters. On 20 meters it will be a 4 lobe pattern with large nulls perpendicular to the wire, maybe as much as 30 dB below the lobes in an ideal configuration, however due to things not being perfect, those nulls might be only about 10-20 dB. There are also nulls off the ends of the wire. On 15 meters the pattern has 6 lobes and 6 nulls. The nulls are very deep, much like what happens on 20 meters. On 10 meters there are 8 lobes and 8 nulls with the largest two lobes coming off the long side of the antenna at 36 degrees to the wire. The nulls are very deep on that band also. Gain in the lobes will depend on antenna height and feedline, balun, and tuner losses.
Let me correct, 40m ( 1,5 :1 ) 20m ( 1,4 : 1 ) 15m ( 1,5 :1 ) 10m ( 1,3 :1 ), these are the right values.
The main question is:
Does the OCF on 1/2 wave of 40m, has better performance then individual monoband dipoles for 20, 15 and 10 m ?
Last edited by PY2CAA; 01-16-2012 at 06:18 PM.
In my opinion any wire longer than a half wavelength is never as good as a half wave dipole as a general purpose antenna. The reason for that statement is that I think a predictable area of coverage is more important in a general purpose antenna. A long antenna will have lots of lobes and nulls and those lobes seem to never be where you would like them to be. For example, if you have a certain direction that is important and you oriented your antenna such that a lobe is in that direction on 20 meters, you may find that on 15 meters a null is in that same direction. With a half wave dipole the lobes are always perpendicular to the wire. It's easy to aim, but of course you need multiple antennas. The lobes for the longer wires usually have more gain than a half wave dipole but that is at the expense of more nulls. If you need only one band and one direction, a longer wire (like an EDZ) might be a good choice because you can get more gain, but that won't be a very good general purpose antenna because it doesn't cover much area. However, not everyone can afford multiple antennas and in that case a multi-band wire can be a useful compromise.