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Author Topic: End Fed Antennas... Which End?  (Read 2221 times)
W7RUE
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Posts: 13




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« on: December 19, 2012, 11:28:26 AM »

A question for for you antenna/electrical engineers...I have several HF, half-wave, end-fed type antennas that I use when traveling or when I am operating "Hotel Portable". They are usually draped over the nearest tree or other handy support from my picnic table or hotel room in an inverted "L", "V", vertical, or sloper and/or a combination of whatever it takes to get it up in the air. There is a matching unit with a coax connector on one end which I usually hook a short piece of coax to and then that to my radio. I then hang/throw the other end over whatever I can find for a support.

QUESTION: Does it make any real difference which end of the antenna is highest? I know that on a typical dipole, most of the signal is radiated in the center of the antenna and the ends can be drooped if you in a space restricted location because there little to no RF radiation on the far ends of the wire. Logic tells me that I should, if possible, get the matching/coax end up higher than the far end. Correct? What say ye? Huh

Regards, Rue - W7RUE
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WB2WIK
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« Reply #1 on: December 19, 2012, 11:34:33 AM »

If it's really a half-wave, it should have the same radiation properties as a 1/2-wave center-fed dipole, even though end-fed.

The real difference is how well the coax shield is isolated from the antenna, which is a big trick with an end-fed; as such, usually some of the radiation is from the coax, as well as the antenna.  If that's happening (likely), then having the cable routed in the most optimum manner can help.

Hotels can be notorious noise sources from all the electronics used therein, in lots of rooms simultaneously.  I'd probably try to arrange the antenna and its feedline to minimize RFI to the receiver!  Picnic/camping spots are probably a lot quieter.
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W7RUE
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« Reply #2 on: December 19, 2012, 12:21:47 PM »

Thanks! That makes sense! I do from time to time, have RF problems from operating from hotel rooms. It is usually the climate control systems that react to the RF, depending on how well shielded things are. (I never thought that climate controls/thermostats could make noise when you key up but they sometimes do!) I operate 20, 40, and 80 meter end-fed dipoles and it is sometimes very challenging to get a full half-wave 80 meter up and out without alerting the management. Being stealthy is half the fun!

Regards! W7RUE Cool 
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W1JKA
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Posts: 1777




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« Reply #3 on: December 19, 2012, 12:32:04 PM »

        No engineer here,just a personal observation from an avid efhw user for three years at various locals and different ground conditions. My best results for a horizontal or sloper configuration are with the matching unit on the high end,and of course low end for inverted  L. Without getting into technicalities, I have read and have been told by other knowing hams that  an efhw should have at least 16 ft. of coax between rig and matching unit for best effeciency.I started out with short coax and/or matching unit direct from rig and thought everything was just fine untill I started using a 20 ft.coax line between rig and matching unit which also doubles as the feed line up to a horizontal or sloper,for inverted L, excess is coiled. I also noticed noise level was down even when I had this extra coax on low end feed of inverted L.The whys and hows I'll leave to the experts,I just know it works for me.It will only take 16 or so ft. of extra coax to see if this improves reults for you.GL  Jim
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WB6BYU
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« Reply #4 on: December 19, 2012, 01:18:16 PM »

Personally, I generally feed an end-fed wire at the end closest to the transmitter.
That seems to work better (and requires less coax) than feeding it at the far end,
especially if you are using a manual tuner to match it on multiple bands.

Then just put it up however you can and operate.

When in doubt as to whether one end should be higher or lower than the other end,
just get them both up as high as your supports allow.


IF you have significant radiation from your coax feedline, then getting it up
in the air higher will help the overall radiation.  But a better approach often
would be to eliminate the common mode current so tall the radiation is
from your antenna instead, in which case it won't matter and you can
arrange it however is most convenient.

If the antenna works better with a longer feedline, it is just reminding you that
the outside of the coax is your ground radial system and you must have equal
currents flowing into the antenna and into the ground system.
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AA4PB
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Posts: 12905




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« Reply #5 on: December 19, 2012, 01:34:44 PM »

EVERY end-fed antenna, regardless of length, requires a counterpoise of some type. Most of the commercial units actually use the coax shield as the counterpoise which is why it works best with a fairly long coax running up to the high point.

Another option is to have the antenna feed point at ground level and connect a bunch of radials to the coax shield connection. That minimizes the coax shield radiation so the coax length won't matter. This is often done with a remote tuner located at the feed point for multiband operation.

In either case, if the antenna is 1/2 wavelength long, then the maximum current and the maximum radiation occurs at the center of the antenna.

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W5DXP
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« Reply #6 on: December 19, 2012, 08:01:59 PM »

In either case, if the antenna is 1/2 wavelength long, then the maximum current and the maximum radiation occurs at the center of the antenna.

Which means that it doesn't matter which end is the highest. The maximum radiation will be from the midpoint between those two end heights.
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73, Cecil, www.w5dxp.com
The purpose of an antenna tuner is to increase the current through the radiation resistance at the antenna to the maximum available magnitude resulting in a radiated power of I2(RRAD) from the antenna.
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