[Yaesu] Single band (80 meters) high swr problem

Fri, 02 Jan 2004 23:30:58 -0500

First off, I think most manufacturers discourage the usage of the 
internal tuner when you are driving an amp - the input circuitry of the 
amp should already be looking like a nice 50 ohm load to your rig - and 
attempting to use the tuner to treat the amp as an antenna system means 
you are asking for troubles!

Try this:

a) WITHOUT THE AMP CONNECTED TO IT

...hook output of the transceiver into a 50 ohm dummy load.

See if it is flat across ALL the bands... or about the same very low swr 
on all the bands (seeking to eliminate xcvr as having the problem). NOTE 
YOU SHOULD NOT BE USING THE INTERNAL TUNER!!!!!!

You are going into a 50 ohm dummy load and absolutely no tuner 
enhancement should be needed whatsoever!

b) after doing this and assuming you find everything is fine (note - 
again the AMP  IS   NOT   JUST   TURNED   OFF   BUT OUT OF THE LINE 
ALTOGETHER!)... put the amp back in the line but do not turn it on, i.e. 
if the t/r relay in the amp is not energized since it is shut off and 
that xcvr rf should be just passing through the amp on its way to the 
dummy load again.

c) IF things still seem fine, ensure that the SWR readings are still 
flat on ALL of the bands, they should vary very little from those with 
the amp not being in between the rig and the dummy load... YOU ARE 
verifying the amp is transparent when turned off...

d) if you turn on the amp and then realize problems, my guess is that it 
is some sort of input circuit problem, but it could be something else....

If it is only one band that is a mess, open up the amp (no power of 
course) and check the path of the RF for that band... frequently a 
switch contact that changes the point where you tap into a coil is 
bad... sometimes the lower frequencies will use coils in series so if it 
is an 80 meter problem I would suspect 160 is also screwed up.

If none of your problems have reared their ugly heads... the rig and 
xcvr are fine - it is an antenna issue, perhaps a bad coax switch wafer 
and while you are in the 80 meter position it may be trying to run both 
the 40 and 80 meter antennas since the wafer is out of alignment or 
somesuch... if you have splices in the feedline try moving the dummy 
load away from the rig a splice at a time to see where things go bad

GL

Jim, K4OJ

...hot off a day of this kind of stuff including rediscovering my 
six-pack still needs more than 12V to function properly....

[email protected] wrote:

> A friend of mine's 1000mp is having difficulties, and I am hoping
> you can help me.
> 
> The problem is this. On 80 meters only (all other bands are fine) when
> transmitting into an alpha 86a amp, without the internal tuner engaged,
> there is a high swr indication that causes a power foldback which does not
> allow the amp to be driven to full output.
> 
> Some additional data points since my first posting.
> 
> The problem is the same with both antenna ports A & B on the MP.
> 
> The rig seems ok in the ssb portion of the band, but the swr
> problem gets worse the lower in the band I go.
> 
> Transmitting into a dummy load results in a flat swr and full output.
> 
> The problem exists with two different alpha 86a's, both of which work
> fine with other rigs, and show no input swr problems with those rigs.
> 
> It looks to me like there is an out-of-band radiation problem with the
> mp causing the high swr/low output problem.
> 
> Unfortunately, I don't have a spectrum analyzer to determine exact
> frequency of the spurious radiation.
> 
> Placing an ICE bandpass filter inbetween the rig and the amp allows
> full output.
> 
> Employing the internal antenna tuner allows the reactance to be tuned
> out, resulting in full output.
> 
> While the later two situations provide bandaide solutions, they are
> not dealing with the real problem which I would like to be able to
> fix.
> 
> I suspect that playing with the line length between the rig/amp
> my bring the swr down, but again, this appears to be just a coverup
> for the real problem.
> 
> Any other suggestions as to what I should look for.
> 
> Have there been known failures of the bandpass filters? If so how
> difficult are they to repair?
> 
> Thanks in advance.
> 
> cheers, Paul - VA7NT ex VE7CQK - email: [email protected]
> "Those who hear not the music. . . think the dancers mad."
> 
> 
> 
> 
> Moderator: Ray Brown, KB�STN
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