On 1/19/2021 8:20 PM, Andy Durbin wrote:
A couple of people expressed an interest so here is a link to a presentation on
my mag loop. It's a while since the presentation was given but the loop is
still in service in the gamma match configuration.
"I wonder if the SWR runaway at 150W could be due to the PVC pipe used as
the insulator for the variable capacitor? PVC is quite lossy at RF.
Perhaps when it gets hot it detunes the capacitor. It would be easy to
check by simply feeling the cap to see if it gets warm."
The insulator/dielectric
I love to see people doing serious homebrew. Bravo!
I wonder if the SWR runaway at 150W could be due to the PVC pipe used as
the insulator for the variable capacitor? PVC is quite lossy at RF.
Perhaps when it gets hot it detunes the capacitor. It would be easy to
check by simply feeling
A couple of people expressed an interest so here is a link to a presentation on
my mag loop. It's a while since the presentation was given but the loop is
still in service in the gamma match configuration.
1 vote for interest!
73,
Fred ["Skip"] K6DGW
Sparks NV DM09dn
Washoe County
On 1/19/2021 4:39 PM, Andy Durbin wrote:
"although I saw a design not long ago that used two coaxial copper pipes with a PVC
pipe as the insulator. The inside conductor was mounted to a long threaded rod moving it
"although I saw a design not long ago that used two coaxial copper pipes with a
PVC pipe as the insulator. The inside conductor was mounted to a long threaded
rod moving it in and out of the outer conductor."
Not my original idea but that's what I have on my loop (PEX insulator not PVC).
Here's an interior shot of the tuning capacitor
https://imgur.com/a/sYdvgzF
-Eric KI7LTT
On Mon, Jan 18, 2021 at 10:59 PM Alan Bloom wrote:
> > That doesn't sound like they are welded, and given the cost
> difference for welded air variables I doubt MFJ used them.
>
> As I said, I don't have
Certainly don't want to throw away power in wasted heat, but when I turn down
my 200W rig to 5W for QRP, it's still useful and that's 16 dB down.
With ant restrictions, I'm looking at building a small horizontal loop for 6m.
It'd be a little more than 4' in circumference, 16" dia, and the
I've been convinced that AEA's specification of 72% efficiency for the
Isoloop at 14 MHz is too high, certainly too high for the MFJ antennas.
So I re-did the calculations using the 59% efficiency figure calculated
below. The new results can be downloaded here:
The plates in the two air variables in the [now ancient] ARC-5 command
TX from WW2 were very securely welded in place to the rotor axle. We'd
remove many of the rotor plates and re-pad it with a fixed capacitor to
spread out the ham band on the dial. It took a lot of bending back and
forth
I got a MFJ 10-30m loop at a SK sale. The original owner had never got it to
work. The rotor vanes had fallen apart. The fixed vanes were welded to the
loop. After re-assembly it worked well, but was difficult to tune as I
couldn't hear the buzzer for finding the tune point, it was too high in
loop quite
interesting too. He uses a KX3 + KXPA100 with his mobile loop.
Warren Merkel, KD4Z
On 1/19/2021 12:17 AM, elecraft-requ...@mailman.qth.net wrote:
Date: Mon, 18 Jan 2021 20:17:38 -0700
From: David Gilbert
To:elecraft@mailman.qth.net
Subject: Re: [Elecraft] Efficiency of MFJ remotely
> That doesn't sound like they are welded, and given the cost
difference for welded air variables I doubt MFJ used them.
As I said, I don't have one so I can't say for sure. I got my
information from the MFJ web site: "All welded construction, no
mechanical joints, welded butterfly
MFJ sells the butterfly capacitors they use. You can see them here.
https://mfjenterprises.com/products/mfj-19
https://mfjenterprises.com/products/mfj-23
wunder
K6WRU
Walter Underwood
CM87wj
http://observer.wunderwood.org/ (my blog)
> On Jan 18, 2021, at 8:54 PM, Alan Bloom wrote:
>
> > There
> There is a reason why top quality variable capacitors often use
welded plates.
I believe they do weld the capacitor plates and also weld the loop to
the capacitor. (I don't have one, but that's what I've read.)
> Yours is a limited theoretical analysis ... not a practical one.
A number
According to another ham who recently posted here, he had to "tighten"
the plates on the MFJ capacitor to get it to work properly. That
doesn't sound like they are welded, and given the cost difference for
welded air variables I doubt MFJ used them.
I sincerely doubt that an actual
You are neglecting the losses in various connections in the system ...
including possibly the construction of the capacitor itself. I don't
believe that they are insignificant. There is a reason why top quality
variable capacitors often use welded plates.
I would also guess that contact
Thank you for sharing that... I have saved it to my system.
73, and thanks,
Dave (NK7Z)
https://www.nk7z.net
ARRL Volunteer Examiner
ARRL Technical Specialist, RFI
ARRL Asst. Director, NW Division, Technical Resources
On 1/18/21 2:05 PM, Alan Bloom wrote:
MFJ makes a pair of small,
Well let's see...
Radiation resistance of a small loop is 31,171 * (Area / wavelength^2)^2
For a loop with a 91cm diameter at 14 MHz, I believe that comes out to
0.064 ohms.
Assuming the loss is due to the RF resistance of the loop:
From the internet I get the volume resistivity and skin
72% sounds way high to me, and I also think it's a big leap in logic to
assume that MFJ quality (materials, connections, etc) is similar to any
other reputable company anyway.
Dave AB7E
On 1/18/2021 3:39 PM, Wayne Burdick wrote:
Hi Alan,
72% sounds a bit high. Is this number based on
Alan,
I think 72% is really optimistic for both the AEA Isoloop and the MFJ.
For a 3/4" copper tubing loop 3 ft in diameter, at 14 MHz, I come up with:
Antenna efficiency: 38% (-4.2 dB below 100%)
Antenna bandwidth: 31.5 kHz
Tuning Capacitance: 85 pF
Capacitor voltage: 2,445 volts RMS
Hi Alan,
72% sounds a bit high. Is this number based on loop size alone ("in theory")?
Or are they taking conductor geometry and other losses into account?
Wayne
N6KR
> On Jan 18, 2021, at 2:05 PM, Alan Bloom wrote:
>
> MFJ makes a pair of small, remotely-tuned loop antennas, the MFJ-1786
MFJ makes a pair of small, remotely-tuned loop antennas, the MFJ-1786
that covers 10-30 MHz and the MFJ-1788 that covers 7 to 21+ MHz. As far
as I can tell, the two antennas are identical except for the size of the
tuning capacitor. Each consists of a 3 foot (91 cm) diameter loop made
of
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