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Author Topic: RF in Shack ??  (Read 3999 times)
WB6BYU
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Posts: 13163




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« Reply #15 on: May 31, 2012, 11:32:44 AM »

Quote from: KJ6TSX

I think I know why my tuner is having so much trouble tuning the antenna. It is trying to tune 3 antennas at once...



Why is that?  Do you have three antennas connected in parallel without a switch?
Or are you switching among them, and the antenna tuner is not retuning for each?
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AA4PB
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« Reply #16 on: May 31, 2012, 12:41:23 PM »

The tuner isn't trying to tune 3 antennas at once. It is trying to match the total resultant load impedance at the end of the coax to 50 Ohms. The three antenna elements are resonant on three separate bands. The one that is resonant on the frequency you are transmitting presents the lowest impedance and accepts most of the power. The others are not resonant on that band and present a high impedance and don't accept much of the power (think in terms of Ohms law and three resistors in parallel).

The antenna is balanced (a dipole with the wires on both sides of equal length) so it shouldn't be sending any common mode currents back down the coax shield to the radio unless it is coupling into the coax shield because it runs parallel to the antenna before it gets far enough away. I think I read that the antenna is 100 feet away from the shack. If so, that' plenty of separation unless it is located close to and parallel to power lines, phone lines, or other conductors that could couple to it and bring the RF into the house.

If one end of the antenna is closer to some conductor or the ground that the other, that can unbalance it and cause common mode currents to flow on the outside of the coax shield. A current balun or choke right at the feed point might help that.

If you really have a maximum of 1.5:1 SWR then you shouldn't even need a tuner. I'd be suspect of a radio that won't deliver full power into a 1.5:1 SWR load. But then, if you have lots of common mode current on the coax shield the SWR meter may not be giving you a correct reading. It would be handy to have an antenna analyzer so that you can tell what the X and R actually is (independent of the radio) rather than only reading the SWR.
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K8KAS
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Posts: 569




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« Reply #17 on: June 07, 2012, 01:01:57 PM »

George you need a good RF shack ground, not a 8 foot ground rod but a good RF ground. A number of radials wires 20 or 30 on the ground connected to a plate near the shack and the braid of copper strap into the shack. Most hams now a days don't know what a RF ground is or why your shack Needs one... A lot of these strange problems go away once you have a nice low impedance RF ground..
Running a KW I cannot even get my field strength meter to register in the shack with my ground (50 --  50 foot radials connected to a 12 by 12 inch copper plate and 1 inch copper strap to the rig and tuner..Marconi knew this over 100 years ago guys...Denny K8KAS..
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KJ6TSX
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Posts: 116




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« Reply #18 on: June 08, 2012, 08:34:58 PM »

Thanks for all the reply's!!
I can tune the antenna in but it is a very sharp In or out. My radio a Kenwood ts 130s it throttles down the output power based on swr. anything above 1.3:1 drops the output to about 25 watts if I get it to 1:1 output is 125 watts. The RF is out of the shack
Thanks
George
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KC4MOP
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Posts: 731




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« Reply #19 on: June 09, 2012, 04:19:36 AM »

George you need a good RF shack ground, not a 8 foot ground rod but a good RF ground. A number of radials wires 20 or 30 on the ground connected to a plate near the shack and the braid of copper strap into the shack. Most hams now a days don't know what a RF ground is or why your shack Needs one... A lot of these strange problems go away once you have a nice low impedance RF ground..
Running a KW I cannot even get my field strength meter to register in the shack with my ground (50 --  50 foot radials connected to a 12 by 12 inch copper plate and 1 inch copper strap to the rig and tuner..Marconi knew this over 100 years ago guys...Denny K8KAS..
George, Consider the RF shack posting, if your problem returns. MFJ makes a box to create the RF ground and it works every time. The ground rod is for electrical safety and lightning protection.

Fred
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