[AMPS] Where to read Reflected Power
Radiodan W7RF
RFpower@radiodan.com
Mon, 27 Sep 1999 17:56:33 -0700
Hi Jim,
I understand the frustration!
I too, have these same problems. Different points on the transmission line
the SWR will vary. Nowadays I just "blast away" into the antenna!
As you know, I use the C4XL and SWR is good on 10-15-20 and CW part of 40M,
However, on SSB 40 or 17/12M SWR is between 3.5:1 and 5.2:1 (7298 KHz).
On all the above bands I use NO tuner, hey I've got a HENRY!
(don't try this with any toy amplifiers) Your HENRYs will take it as well.
I don't recommend 5:1 SWR with full output in RTTY mode but anything short
of that will be no problem (for the amplifier OK, just be careful of the
high RF voltages present at the feedpoint of whatever antenna has the high
SWR on it).
On 80/75M I use a random length dipole (whatever you can fit and cut it in
half, feed at the middle) fed with 450 ohm ladder line to a VERY heavy duty
AMIDON 4:1 balun just outside my window (RG213 coming in from balun to tuner
about 10 feet of cable). The reason for the Amidon balun is because the
pieces of junk MFJ 989C and Tokyo Hi-Power labs HC-2000 tuners I use can't
take big power through the pitiful little built in balun on the low
frequencies (80/160M). I discovered that they would accept full amplifier
power, 2000+ watts from my 3K Premier, when using the external balun trick
and connecting to a regular SO-239 coax jack on the back of the tuner.
Problem solved for me. Before anyone suggests I try different lengths of 450
ohm line, been there tried that, it was better and worse not perfect.
>From your MFJ 259B readings, it looks like you're off from 50 ohms resistive
and have quite a reactive component.
To sum up, I would tune for what the tuner likes the best IF you are
interested in running big juice and need the tuner to play well under high
power conditions.
Otherwise, either skip the tuner as I do or tune for the amplifier having
the best SWR if your tuner can take whatever power you wish to apply.
Hope your balun and antennas are up to it all. Also be VERY careful not to
fry your 259B by poor isolation on that antenna switch!
73, Dan Magro W7RF, (President WARC 1999, member SCDXC, SCCC)
Manufacturers Rep & Distributor for HENRY RF Power Amplifiers.
RF Applications Power Measuring equipment, See the VFD!
www.radiodan.com <http://www.radiodan.com> RFpower@radiodan.com
<mailto:RFpower@radiodan.com> A trip to our web site is worth the click!
> -----Original Message-----
> From: owner-amps@contesting.com [mailto:owner-amps@contesting.com]On
> Behalf Of Jim Reid
> Sent: Monday, September 27, 1999 4:53 PM
> To: amps@contesting.com
> Subject: [AMPS] Where to read Reflected Power
>
>
>
> Hi,
>
> Was tuning up for some CW on 10 meters today. Decided
> to also use my linear, Henry Ultra. This amp comes with
> a Bird coupler on the back of the RF deck. However, I
> have a 4' length of RG213 on the output connector from the
> deck which runs to a low pass filter, then another 3 ' of
> RF 213 runs back up to the Bird coupler assembly on the
> back of the deck. From the Bird, I run another 10 feet or so
> to a Hofi rotary switch which allows me to switch an MFJ-
> 259B to the antenna to adjust the antenna tuner for good
> match. On my antenna tuner is another "cross needle"
> meter.
>
> What I noticed is that when I have the antenna tuner set for
> almost perfect match using the 259, then switch to the
> rig set up, I read reflected power at the Bird coupler on the
> back of the RF deck, but none on the antenna tuner meter. Or,
> I can adjust the antenna tuner so that there is no reflected
> RF power read on the back of the RF deck after the LPF,
> but read around 50 watts reflected now at the antenna tuner
> meter?
>
> This is on 10 meters, and I also found that I could not get the
> antenna tuner to match to my C4 beam until I had added a
> 3' length of 213 to the feedline coax coming from the antenna
> to the back of the tuner! This tells me that on 10 meters one
> is liable to read about anything anywhere in the set up when the
> antenna does not match the feed system exactly. I have no
> idea what the impedance up at the antenna actually is, but
> with the 259B switched in, and the tuner bypassed, the
> MFJ unit says the impedance is R=22, X=19, vswr=2.6.
> With the tuner set to read essentially 0 reflected power
> on the Bird at the RF deck, that is with the tuner now in
> series with the antenna analyzer, the analyzer reads
> R=45, X=24, vswr=1.6, but tuner switched to the antenna
> now, there is about 50 watts reflected read on the tuner meter,
> as would be expected with other that R=50, X=0 or close to
> those numbers. But, as I said, the Bird on the back of the
> RF deck is reading essentially zero reflected into the
> output of the amplifier terminals.
>
> So, do I want to set the tuner for minimum to zero reflected
> power at the output of the linear, or zero reflected power on
> the back of the antenna tuner, where, presumably, the
> antenna tuner meter is reading?
>
> Seems to me, I want zero or near it, reflected on the output terminal
> of the linear, right? And, as I have not noticed this on
> the lower bands, maybe this is just a 10 meter situation I
> have stumbled upon with my set up here.
>
> Will experiment later on the lower bands, out of time for this
> today, hi.
>
> 73, Jim, KH7M
>
>
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