question about my antena tuner

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RFnewbie

Joined Mar 11, 2021
1
Hi guys! I'm new here and have a question about my new MFJ-971 Antenna tuner I just connected up last night.
In learning to read the swing needles, I have a basic understanding of forward power and reflective power and the standing wave ratio where the needles cross. If I am posting this in the wrong place please forgive me.

So I set my antenna tuner to 6 watts first using the jumpers. Then I notice that before I have connected my homemade antenna to the 50 ohm coax from the tuner, the swing needles were already reading at about the 3rd hash down, say just over 100 milliwatts of forward power. Why is this when there is no antenna connected?

And then next when I take off the 50 ohm coax from the antenna out, the forward power needle drops by half.

So the question is, how can it be reading forward power without a closed circuit and just an open circuit transmission line on the coax?

Then once I do connect my homemade antenna circuit, do I deduct the forward voltage that it was already at before I connected the antenna, from where it is after the antenna is attached, to calculate forward power?

Thank you for helping out an RF Newbie..
 

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nsaspook

Joined Aug 27, 2009
8,786
A open transmission line is not an open circuit, it's a mismatched RF circuit. ;) Forward power out, reverse power back with meter readings depending on where inline the tuning measurement pickup is. If connected to only an antenna the diode power detector section could become a wide-band RF receiver on a unloaded transmission line.

MFJ971-User-Guide-Page-4.png

 

sagor

Joined Mar 10, 2019
491
At a very low wattage setting, your device may be acting as a radio receiver and the RF it gets through the coax/antenna is being rectified via the diodes. Is there any TV or radio broadcast stations within a few miles of you? If so, that is what is happening, most likely.
That said, I would expect the reflected meter to show that RF, not the forward meter. But then, there could be other similar reasons such as RF noise nearby in the house, etc.
Change the length of the coax, does that make a difference? A certain length can act as an antenna for specific frequencies.
 

ronsimpson

Joined Oct 7, 2019
1,549
how can it be reading forward power without a closed circuit and just an open circuit transmission line on the coax?
A 1/4 wave length, 1/2 wave length, 1 wave length piece of coax has very different properties. (also any multiple of) A piece of coax can be seen as a open or short depending length verses wave length.
 

Yaakov

Joined Jan 27, 2019
3,603
Remember, your tuner is trying to match the transmitter’s output to space using the transmission line and antenna. So don’t think of the antenna system as the load (though it can use power heating up, which is just a loss), think of it as the connector that plugs the transmitter into space. Power is reflected when the antenna system Is poorly matched but some power is radiated and makes it out, so to speak.

So, unless you manage to reflect 100% of the input power, some power will be used in the forward direction and will be indicated.
 
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