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crimso Profile
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Registered: 08-2009
Posts: 92
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End fed wire.


First thing to say is. These antennas work best were the length is optimised for the band (or bands,) in use & where an earth is fed against.
These are not great antennas when compared to some & for a variety of reasons, but don't write them off.
Good points are simplicity & cost, bad points can be RF in the shack, RFI problems & inefficiency.
I have used end fed wires for years, in a variety of locations & in a variety of configurations & have had good results with all.
I have one random wire fed via a magnetic balun. This works OK at higher frequencies but is very poor at frequencies below say 4 MHz. But it's a reasonably short wire with no earth.
The magnetic balun allows me to reduce electrical noise at the RX, gives me electrical separation between the coax feed & the antenna & thus it also lowers the chance of RF in the shack, but a ugly balun (RF choke,) at the feed point helps eliminate any RF coming back down the coax.

My other wire is about 80 feet in length & fed against a simple electrical earth stake & via a balun, or to be more accurate an un/un.
I use no RF choke on this antenna.
One thing I would say is ALWAYS use a good ATU with wires, as the current can be high & thus flash over is a problem worth bearing in mind.
3/Sep/2009, 5:07 pm Link to this post Send Email to crimso   Send PM to crimso
 
John193 Profile
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Freebander
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Re: End fed wire.


OK there seems to be a major theory problem here my friend.
 I am going to take the time to elaborate here because if this is read and accepted then a lot of readers are going to go away being grossly misled.
To start with, there SHOULD be a very LOW CURRENT on the feed point of an end fed antenna, because this feeding method is known as VOLTAGE FEED. What you should have is a HIGH VOLTAGE. This is because you are feeding the antenna at a voltage max point NOT a current maximum point as in centre fed antennae. If the wire is a harmonic length then it will be voltage fed and this is the type used in most base 11m verts, ie Antron, Sirio 827,imax and so on and on (they are all end fed wires).
You certainly DO NOT need a tuner to drive a Sirio or Antron any other end fed vert or horiz, and you will throw away dB for nothing if you do this. You certainly do not ground these types of antenna that will surely earth out all your hard fought signal, they drive against an unbalanced load and thus the coax braid is the earth or ground image. Most ground image systems in use are counterpoise type and are very much superior to a ground spike anyway ie they are hamonic also which helps efficiency further. I hope i haven't upset you here but your theory on end fed antennae is a little wayward.
While on this point of grounds a counterpoise/groundplane or to use its correct definition image plane is an attempt to establish a lower impedance earth image in order to improve the overall EFFICIENCY and has no part in harmonics it is purely to lower the brewster angle of the lobe in an attempt to replicate a perfect earth conductor.
The coil in the base of many antennas of this type is to match the antenna at over a 1000 ohms to the coax at a mere 50. As this coil is connected to the feeder, normally coax it is made of heavy material to reduce losses and as it transforms up the high current becomes instead a high voltage and is thus then connected to the radiator and all the energy is radiated (in a perfect world). If you were to feed two end fed antennas from a parrallel line feeder then you would not require a coil or balun because being twin feeder the impedance would be similiar to that of the end fed wire's impedance a good match would result and again the rf would be almost totally radiated.
 For clarification on this point you can feed a 1/2 wave radiator from ANY point along its length ie not current max nor Volt max and this sytem is used in many matching systems like gammas, Tee and clemens matches.I am not attempting to piss on your fire here truly, but I have many years instructing on this subject, and you have wondered into my speciality subject lol.
I would agree with you on some points if you were talking about a long wire or marconi(which is not always end fed)which as you know is more than five lambda in length to qualify as a long wire and can be tricky to feed correctly due to its very high impedance (5000 ohms+)but thats a different subject again.
 I hope this has clarified the subject of end fed wire antennas somewhat and as most of the antennas in use as verts on 11m are end fed antennas and anything in GRP is generally a wire I would dissagree with the statement that they are innefficient, I have tested some end fed full length wire ants at 98% and most commercial ones for 11m are around 95% that is certainly not innefficient in the real world, that would be the case if you are using it non harmonic as any antenna would be but efficiency is just radiated Power/supplied power x100.
The choke and or balun you mentioned is another gospel so I won't get in to that one right now. The flash over you mentioned is not a current issue at all but a voltage one, it is the high voltage jumping the insulation or insulation breakdown and it matters not whether the current is 1000 amps or 1 amp it is the voltage that needs to be high in order to break down the insulation in the tuner whether the insulation is teflon or air gapped. The current will only become an issue if the arcing is caused by surface contact and then high current will flow and create a real torch like arc this can then weld itself to death. I hope that clarifies and hasn't muddied the waters too much.
By the way is the 80m wire for use as a half on 160m?

Best regards
John 193
28/Sep/2009, 11:06 pm Link to this post Send Email to John193   Send PM to John193 MSN
 
VC7 Profile
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Freebander
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Registered: 04-2009
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Re: End fed wire.


Bloody hell John he was only trying his [sign in to see URL]

regards
VC7 Jon

25/Oct/2010, 7:43 pm Link to this post Send Email to VC7   Send PM to VC7
 


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