On 11/28/2010 4:52 AM, Kevin Normoyle wrote:
> Good ARRL article on tuning t-network tuners
>
> http://www.arrl.org/files/file/Technology/tis/info/pdf/9501046.pdf
>
> I was going to say, based on my limited experience, than you shouldn't sweat
> the
> L vs C choice, and just aim for a 1:1 swr
> My thinking: if you're losing at most 1 db in the tuner, your feedline and
> probably balun losses and ?? other losses dominate over that.
Under normal circumstances that would be a valid assumption.
> But Figure 3 was interesting in the article. The min-max curves show the
> range
> of loss matching into a 10 ohm load...80M showing a>2db loss.
Well...kinds sorta. Remember that will larger capacitance you can drop
the loss to to around 0.3db (Fig 4), but they won't be air variables.
OTOH if you want a good tuner like a good amp, go for good components
and pick up a couple of vacuum variables off e-bay or other sources. You
can pick up new ones for the input and output for maybe a couple hundred
dollars total. So you end up with 3 turns counters instead of one.
Even with air variables the high loss occurs with low antennas and low
frequency so they could be considered almost special cases.
Wire antennas work fine, but all antennas are a group of compromises.
Typically the more bands an antenna covers the less efficient it will
be. A G5RV is a good compromise antenna. Not great on any band, but it's
a good overall antenna specially considering limited space and limited
budget.
At only 30 to 35 feet high it's not exactly the best for low angle DX
but many have worked a lot of DX with them.
Me...51 years and I don't even have WAS and that's even after having a
contest style set up with separate stations and antennas on each band.
Course I never tried to make WAS so that might be part of it. Contests
and FD. I jump right in...and get bored in about 15 minutes.
I like to experiment...and talk. I did make RCC while still a Novice. <:-))
73
Roger (K8RI)
> Now back to MFJ bashing.
> I used a Palstar at-1500cv in the past, and went back and read up on it. Even
> Palstar reduced their mx power recommendation when feeding low impedances.
>
> So: if MFJ failed at the design job, the real question is: how much power is a
> "good" tuner supposed to be able to dissipate without catching fire?
Looking at the derating on 160 and 75 depending on the antenna height,
even the big commercial tuners are down to around 500 watts max.
> That ARRL article shows a case with 40% losses in the tuner.
>
> Is the only good tuner one that can dissipate 600 watts?
> With a fire, I'm assuming we're talking about coil dissipation and not
> capacitor
> arcing.
Cap arcing on 160 is quite common even with a few hundred watts. My old
989C tuners used to hiss at me like a snake every once in a while. You
need either wide spaced caps, or vacuum variables to run power if the
antenna is a bit long. I've not arced the AT5K yet, but I haven't had
it on 160 at the legal limit yet either.
> Rather than just bashing vendors, it would be nice if people complained more
> precisely: i.e. a tuner costing XYZ should be able to tune ABC with DEF watts.
>
From my experience, a tuner that is going to run the legal limit into
any practical, low 160 and 75 meter antenna would cost a fortune, use
vacuum variables, and a very large rotary inductor with edge wound
strap. All component spacing would necessarily be large so the over all
size would be ...big Now you can play with the height and length and
get rid of that pesky arcing, or at least reduce it.
> Then we could debate if it's possible to achieve those goals for that cost. If
> so, is the argument that MFJ is pocketing obscene profits?
> Or: that people don't know what they need?
>
> -kevin
> ad6z
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