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Author Topic: Yaesu FT897D+FC40 RF problems  (Read 1451 times)
GOUDURIX
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Posts: 204




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« on: January 27, 2007, 03:43:56 AM »

I have installed Yaesu FT897D and FC40 ATU in my QTH shack. Rig is destined to be installed in a boat later with occasional use at home.
I made an Inverted-L wire antenna about 8m above ground, 11m of wire up from atu-connection and about 11.5m horizontal part. For RF ground I have 3m copper ground rod, and several 20m and 10m ground radials connected also to ATU ground lug. The ATU is inside shack against outer wall, only 0.5m from ground rod.
I have about 10cm of the end of antenna wire coming inside through stone wall and connected to ATU. The tcvr is about 2 meters away from ATU, has also heavy gauge ground connection directly to ground bus bar.
On an inverted-V dipole for 40m I have perfect SWR and very good modulation. On the inverted-L, reports say 'clipped modulation' on 40m. On 80 meter on the inverted-L, the ATU provides good tuning (VSWR < 1.5) but I have RF-feedback to the tcvr, provocing the tcvr to start up some menu controled functions or switching off the tcvr.
Any suggestions or similar experiences?
Jan from Belgium
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K8AG
Member

Posts: 349




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« Reply #1 on: January 29, 2007, 06:35:39 AM »

An inverted L is really a quarter wave antenna which is simple bent over part way up.  If it is a 1/4 wave antenna on 80M it is close enough to 50 ohms impedance that the tuner can easily match it.  But that means that on 40M it is pretty close to a half wave radiator.  It will exhibit a very high impedance (perhaps a thousand or more ohms) to the tuner.  Even the best autotuners can have a very difficult time matching that.

There are tuners that can adapt very high impedance, but autotuners usually are not of that type.  You might try an N:1 transformer to drop the impedance down to a reasonable level so the tuner can handle it on 40.  But then the transformer would need to be removed for 80M operation.

If you have the space you might consider a G5RV or even a ZS6BKW.  These have specially designed sections of window line that provide transformations and allow better SWR on 80 and 40 M (as well as other bands).  You still will need a tuner, but you avoid the whole 1/4 wave on 80M is 1/2 wave on 40M problem.

Hope this helps.

73, JP, K8AG
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WW5AA
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Posts: 2088




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« Reply #2 on: January 29, 2007, 11:23:47 AM »

Jan,

I would first ask if you are using a 1:1 balun ? A coaxal balun would work fine to keep the RF out of the shack. With a ground mounted 1/4 wave vertical (verses raised) the radials should be as long as possible (and as many as possible). I would connect the radials to the ground rod rather than the ATU and run ladder line, one fed to the antenna and the other to the ground rod. I believe you will see a big improvement.

73, de Lindy
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GOUDURIX
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Posts: 204




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« Reply #3 on: January 30, 2007, 05:31:24 AM »

TNX for your replies.
Yes I am now using an RF choke rather than a strict balun. It is 21 feet of RG-58 coax coiled around an 11 cm-diameter PVC tube form, attached at the Coax entry (before the atu) and going back to the tcvr with about 2m of coax. This is not helping for the RFI on 80m.
I will try to install ferrite toro├»dal core based RF chokes instead  on the coax + on all other leads in and out of the tcvr (ATU feed line, Mic, headphone).
If this doesn't help, move the ATU outside the shack.

Could you elaborate a bit on the ladder line feed suggestion (how, why)

TNX in advance,

Jan
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WW5AA
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Posts: 2088




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« Reply #4 on: January 30, 2007, 06:36:37 AM »

Jan,

Your coax balun is too long at 21' and may be part of the problem. 10-6" turns is all that is needed. It seems you are running a single feed from the balun to the antenna ? If you run ladder line from the balun, center to the antenna and shield to the ground rod I believe it would work better. Then try disconnecting the ground wire to the tuner and see if this helps. As said before the radials should be connected to the ground rod. Hope this helps.

73, de Lindy  
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GOUDURIX
Member

Posts: 204




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« Reply #5 on: February 04, 2007, 03:07:33 AM »

Hi de Liny, I will try to explain better.
I have about 78 feet of inverted-L, halp of it horizontal at 8m high, and from there half of it vertical down untill the ATU wire antenna connection. The lower end of this feed wire (which is the end of the inverted-L) goes through the house wall into the shack. The Yaesu FC-40 ATU is mounted against the outer wall, and about 10 cm of the wire comes into the shack. ATU ground is connected to heavy ground T-bus bar; from there 0.8m heavy copper lead to ground rod. 6 radials (1/4 wave cut to 80m, 40m, 20m each) are connected directly to ATU ground, parallel to the ground rod connection (I was told so). Between ATU and tcvr, very close to ATU, is the RF choke made of coiled coax. The 20 feet length I found in a article on the net (20 ft of RG58 closely wound on 10cm PVC core). This is about 18 windings in my case.
In another link I found the advice for 3 to 4 meter coiled on large diameter PVC core.
Could you elaborate on this?
BTW I have just solved the RFI problem by installing ferrite chokes on ATU feed wire, Mic wire and removed the coax RF choke and replaced by 2 turns of heavy coax feed through ferrite core as close as possible to ATU (and between ATU and tcvr). The RFI problem on the tcvr at 80m and via the inverted-L has gone.

I am interested in more on the coax RF choke, as I just installed a Fritzel FD4 Windom antenna (for 80-40-20-10m) as invertedV, and want to be sure that I avoid all RFI/TVI, so maybe a coax coiled choke just under the 1:6 balun might be advisable?

Jan
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