[Moon] Antenna, 8877 amp SWR

Leif Asbrink leif at sm5bsz.com
Mon Nov 5 21:59:56 CET 2018


Hi Roger and Serge,

if Roger has the correct explanation, running at 25%
of the power should make the process 4 times slower.
That would be easily detected.

Another test: If your antenna is shorted at DC (which
it should be to protect your LNA from damage due
to static rain) you could send an appropriate DC
current through the cable for a couple of minutes
before the first transmission. 

A non shorted antenna could get chargewd to kV levels
in static rain. Switching in the LNA could cause a
spike that would kill the LNA. If your antenna
is not DC shorted, add a shorted 0.25 wl cable
on the antenna side of relays. Best use 70 ohm cable
which has lower loss than 50 ohm cables.

1kW is 4.5 A RMS. Losses at 144 are much larger than
losses at DC, but connectors and everything should be 
perfectly happy with 5 A DC - and if you run that for
a couple of minutes, heating should be larger than
half a second at 144 MHz.

73

Leif



> Serge,
> That sounds like a slight amount of moisture vaporizing on initial high
> power transmission and condensing again when allowed to cool. Probably at a
> low spot or loop in the feedline.
> 
> 73, Roger
> W7TZ
> CN83ia
> Grid Busters
> w7tz.webs.com
> 
> 
> On Mon, Nov 5, 2018 at 6:47 AM Serge Szpilfogel via Moon <
> moon at moonbounce.info> wrote:
> 
> > Thank you Gaby & all for detailed information's. As soon I get a dummy load
> > I will find out FIRST if the problem is in the radio room or at the
> > antenna.
> >
> > What makes it difficult it only happens on the first transmission of the
> > day
> > for less of a minute then it is ok for hours.
> > My MFJ analyser for VHF does not tell me anything since you need high power
> > going through the system to create the problem.
> > Thank you to all with good suggestions.
> > Serge VE1KG
> >
> >
> >
> > -----Original Message-----
> > From: Draskovits Gábor HA1YA [mailto:ha1ya at isiscom.hu]
> > Sent: November 5, 2018 07:41
> > To: 'Serge Szpilfogel'
> > Subject: RE: [Moon-Net] Antenna, 8877 amp SWR
> >
> > Hi Serge,
> >
> > Leif has right!
> >
> > The cause of your problem 99% high level spurious or harmonics level in
> > output power.
> >
> > Easy way to check it:
> >
> > Put with short cable 50Ohm dummy to the PA and check the VSWR.
> >
> > If you have spectrum analyzer you can see the harmonics or spurious sigs in
> > the output power.
> >
> > If you’ve tetrode amplifier and its self oscillating anwhere:
> >
> > Check the G2. If the G2 grounded  capacity is broken or not enough the tube
> > can generate similar problem.
> >
> > For example at GU87B or GU84B if only one connecting plate on G2 part
> > dosn’t
> > connecting  to G2 body the tube
> >
> > will self oscillate!
> >
> > If everything is OK around the socket:
> >
> > At tetrode amplifiers put pre-dummy to G1 to ground.( 100 Ohm 50W, non
> > inductive serial with 1..2nF capacitor.
> >
> > It will helps a lot. The input stage will little bit wider  and it give
> > stability for the amplifier..
> >
> > …after few 100 amplifier building, with 73,
> >
> > Gabi HA1YA
> >
> > www.ha1ya.hu
> >
> >
> >
> >
> >
> > From: Moon-net [mailto:moon-net-bounces at mailman.pe1itr.com] On Behalf Of
> > Serge Szpilfogel via Moon-net
> > Sent: Sunday, November 4, 2018 10:19 PM
> > To: moon-net at mailman.pe1itr.com; moonbounce reflector <
> > moon at moonbounce.info>
> > Subject: [Moon-Net] Antenna, 8877 amp SWR
> >
> >
> >
> > Hello I have not resolved my 1.5 SWR at the beginning of a TX first time
> > for
> > 30seconds.
> >
> > I changed some connectors & adapters but no results. I am know focusing on
> > the amp & the brick & the transceiver & am looking for a dummy load which
> > can stand a Kilowatt.
> >
> > Question: could it be a spurious frequency generated by the TX or brick or
> > Amp???
> >
> > When I get the dummy load I will be able to check the amp & the brick & the
> > transceiver.
> >
> > I have a DCI 145  Band pass filter which I could put between the brick &
> > transceiver
> >
> > Or the brick & the amp input I guess. Or simply put up with it???
> >
> > Your suggestions are always appreciated.
> >
> > Thank you
> >
> > Serge VE1KG
> >
> >
> >
> > <
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> >
> >
> >
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