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Author Topic: Yaesu FT817 and MFJ 1786 high-Q tunable loop anten  (Read 1430 times)
WA2DTW
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Posts: 91




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« on: November 21, 2001, 07:46:32 PM »

Has anyone successfully used the MFJ 1786 high-Q loop antenna with the FT817?
It requires some RF in order to tune properly.  But mine doesn't.  And when I try to listen with another receiver on the transmit frequency, I hear practically nothing.  I'm wondering if a) the 5 watts is too low power to drive the tuning device and b) before the tuning is complete, the FT817 sees a too-high SWR and shuts down the RF output.
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W5WJP
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Posts: 157




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« Reply #1 on: November 30, 2001, 11:10:07 AM »

I tried a friend's(big QRPer) homebrew loop last field day on 20m PSK31. Performance was disappointing, not as good as the performance of my MP1. The friend uses his loop for QRP CW and says he has no real problem with it. I think his rigs are mostly MFJ CW QRP rigs or homebrew QRP. As for the tuning, we did not have any real probelm tuning it. We used a MFJ 259 to tune and then I used a Z11 after that.
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ADAM12
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Posts: 28




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« Reply #2 on: December 09, 2001, 09:52:14 PM »

I used an MFJ-1786 with an SG-2020 at 5 watts for a while and I was well pleased with the results.

The 1786 only needs a few hundred milliwatts or a watt at most for tuning. First-off make sure you have the 30 watt meter setting set (not the 300 watt setting).

The procedure is to tune first (via up/down buttons) for maximum motor noise heard in receive (go back and forth (up/down) until the capacitor drive-motor noise is at it's loudest through the rig's speaker). Once you have reached this point, you should be in the ball park as regards SWR. You then transmit a CW or AM carrier. The cross-needle meter on the 1786's control box should now show a reading and you should then fine-tune for lowest SWR using the up/down buttons until you get minimum SWR. That's all that's to it.

There is an "auto-tune" mode but it's not in fact an automatic tuner and "auto-tune" doesn't work that well anyway. Use the procedure I've outlined above instead.

Also make sure that you only use the ungrounded power supply that comes with the unit (or internal batteries) and that you have no antenna switches or anything else between the control box and the loop or damage to the control box can result.

When my 1786 was new, I couldn't get the SWR down so opened up the casing and changed the shape of the driven loop (the small inner one) from an elipse to a circle. SWR could then be set to 1:1 on all bands.

Persevere, they work great !
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W2DI
Member

Posts: 133




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« Reply #3 on: September 02, 2002, 08:08:46 PM »

I had a 1786 and was very happy with it. I used an arrangement that didn't require any power from the transceiver to tune it. There are devices on the market that use the principal of a noise bridge to tune an antenna. I have used both the Palomar and Tentec (kit) units and they work well. MFJ also has a similar unit. You first tune the 1786 for max noise, putting you in the neighborhood. Then you turn on the noise bridge and fine tune for a null in the noise. Turn off the bridge and you have a nearly perfect match, without transmitting at all! Something to consider. Best of luck--Joe W2DI
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