Referring to the parallel resonant transformer (especially the excellent description by Chuck N1KGY), if I wanted to put the capacitor on the low impedance side instead of the high side, let's say at 10Mhz which value of reactance should be on the low side? 5.5uH?

I've never had my uBitx on phone.? CW only. I do have a microphone but I'd have to look for it!

Please disregard my previous post as it was a comment on another thread!

What you ask is a tunable pre selector. Simple schematics may be taken from the receiver preselectors by the MFJ. Take the components such as capacitors able to the voltage required. And see whether the insertion losses are acceptable. Besides external LPF (schematics on communication-concept site) I have done a rudimentary variable LPF with just a variable 2*450pf a T-130-7 and a six position switch. For what I can see with my poor instruments (cheap antenna analyzer, AD8307) the variable LPF seems not too bad, considering the problem is beyond the third harmonic.

Thanks Allison, haven't checked the coils yet.? Finally got the rig back on line and operating.? I put in an inline fuse and blew four of them adjusting the bias and rv1.? The VERY best advise I have seen in the group website is the addition of the fuse.? If it helps anyone else I found through monitoring the current draw while adjusting the bias my current draw would climb on cessation of transmitting and could easily blow the fuse......or fry my irf510s.? I found by experimenting and blowing four fuses that the setting of the bias had to sync with RV1 or the current would jump on completion of transmission.? Finally found a point of balance.? I don't know if this is idiosyncratic to my rig or experienced by others.? Conrad's advice about q90 wax also spot on I have bough a few spare 2n3904s and add posts to mount them as I find it difficult to work with the smd pads with a soldering iron.

Cheers Jeff
VK3GMO
ex WA2GMO

Hi Chuck, indeed I was thinking with the logic of building wideband transmission line transformers which cannot be easily built with such a high transformation ratio. As far as I understand you would put the capacitor at the antenna side. If I wanted to put the capacitor on the low impedance side , let's say at 10Mhz, what would you do? Which value of reactance should be on the low side?

I spent the first 10 years as a ham QRP and worked the world and I was lucky to have a dipole and vertical (vertical worked better). Now 40 years later I'm getting back in and going QRP w/ EFHW.?

开云体育

Be very careful playing on 160m in the uBitX stock configuration. ?The PA easily puts out over 30watts with the second and 3rd harmonics to join the party as well. ?You should have a fuse in-line to stop the fets letting out the smoke. ?There is no ALC to tame the amp at low frequencies. ?Plus with the small heat sinks the fets go into melt down quickly.

The problem with relying on tuners to cut harmonics is it is hit or miss. I have a L network tuner I use. Sometimes, on some frequencies, my antenna is in near prefect tune and the matching box is basically passing the transmitter output direct to the antenna. In that case the L network has no chance to act like a low pass filter and remove anything. Maybe there are other tuners that stay in circuit, even when the antenna is a match, with some resonance to possibility act as a band pass filter.?

Tom, wb6b

Would something like this on the output help?

https://www.google.com/url?sa=t&source=web&rct=j&url=https://www.arrl.org/files/file/Technology/tis/info/pdf/8809017.pdf&ved=2ahUKEwjGlvW8m4zdAhUn54MKHSfJCSQQFjABegQIBxAB&usg=AOvVaw3uTeG3YM8UtUS_8W-P5N5V

Bill K8UH,

I think part of the problem here is many get posting one by one in the Email
and from moment to moment they have at best a vague notion of whats
going on till it grabs there attention. The problem then information and
postings are history that has gone past them.

I seriously wonder how many see this in the web format where scrolling
back is as easy as the mouse wheel.? ? Even with the web form if it not the
top 20 active threads it goes into the second of nth page and gets lost there.

Its why stuff gets, needs to be repeated so often.? Maybe the wiki function need to??
be populated.?

Allison

John:
Unfortunately, my uBitx Raduino died,and i haven't had the time to fix it and test my circuits. I should get to it this week.However the simulations are clear, it will work. The fix though is to change the impedance to 25 ohms, of the filters, which allows the capacitor values to triple.That reduces the effect of the relay capacitance. The output transformer needs to be rewound, the filters changed, and a 1:2 transfomer added to get back to 50 ohms. That is the only artwork change, the transformer.

Howard
Quoting John KC9OJV <greusel@...>:

I'll check the finals. I have some spares, just in case.

I'd love to be able to control the power out on the uBitX without opening it up and turning a trimmer pot. If it had a control for output power, I'd use it. The stock machine, as I'm sure you've noticed, doesn't have a way to reduce the output power.

So it is what it is. Perhaps I was naive that it wouldn't blow up while trying to tune for a few seconds.

Based on what you have told us.? Beats us.

Likely finals.
First without a load assure yourself the power supply is ok.

The easy test for fried finals is pull the wire that power them
(and them only). I think it s brown.

Also hint if you are running more than 10W on any band, surprised
you should not be when heat gets them.

Allison

On Wed, Aug 1, 2018 at 10:24 AM, iz oos wrote:

I would use ferrites with permeability close to 800, not the powdered iron toroids like the red and the yellow you have selected (permeability is less than 10).

For a parallel-resonant transformer, we don't want an excessively high permeability; what we want is the core material which will provide the highest Q at the frequency(ies) of interest.? The high-side winding impedance should present a reactance which is near the geometric mean (gm) between the 50 Ohm input and the? Antenna's feed impedance at its end (1500~4500 Ohms).? A 49:1 impedance transformation (7:1 turns ratio) presents a nominal impedance at the antenna terminals of 2450 Ohms as has been previously stated, and for this case the gm is ~350 ohms.? The reactance of the coil in the parallel circuit should be equal to the gm on the lower/lowest band of intended use -

I.E. a T80-2 (red) toroid with 36 turns #24 = 7.13uH.? The reactance at 7.0Mhz is ~315 ohms... close enough for our purpose.? On 40 meters a capacitance of between 65pF and 75pF will resonate the matching unit across the band, assuming the antenna wire is resonant (presents zero reactance) at ~7.100Mhz.
On 30 Meters, the same coil will present an inductive reactance of ~450 ohms, and require 32~38pF of capacitance to resonate the system, assuming the wire is resonant (again, zero reactance point) within the 10,100 ~ 10,150Mhz band.
So, a single -Resonant- EFHW tuner can be used (with separate, resonant antenna wires) on two adjacent amateur bands, I.E. 40 & 30 Meters.

On 20 Meters, the reactance of the coil is on the order of 630 ohms and required capacitance on the order of 18pF, and both of these figures present difficulties in actual implementation.? In the first issue, a gm of 630 Ohms suggests an antenna impedance of well over 6000 Ohms, which is un-achievable in nearly all terrestrial environments; the second issue is that the 18pF figure is inclusive of stray capacitance, which may be 10pF (or more) depending on materials and physical construction...? this will make tuning on 20M very sharp, and offer only a very limited tuning range because the minimum capacitance of the variable capacitor may well be equal to the capacitance necessary to resonate the antenna system.

Having two separate [switched] coils makes it possible to cover 4 or 5 bands with a single matching unit - 40/30 Meters on Coil #1, and 20/17/15 Meters on Coil #2.? Two wires will? cover four bands: A) 40/20 Meters, and B) 30/15 Meters...? a third wire is required for 17 Meters, if desired.

The gm for 40 & 30 Meters is 8.33Mhz, so the T80-2 coil should have a reactance of 350 ohms at this frequency; which is 35 turns on a T80-2 toroid.? The total capacitance necessary to resonate the system at 7.0Mhz will be in the range of 70~80pF, and at 10.12Mhz will require 30~40pF to resonate the system, minus whatever stray capacitance is present.

The gm for 20~15 Meters is ~17.52Mhz, so the T80-6 coil should have a reactance of 350 ohms at this frequency; which is 27 turns on a T80-6 toroid.? The total capacitance necessary to resonate the system at 14.0Mhz will be in the range of 36~44pF, at 18.13Mhz in the range of 20~28pF, and 15~20pF at 21Mhz, minus whatever stray capacitance is present.? The 15pF figure at 21Mhz is very low, indicating that tuning on 15 Meters will be rather sharp, that tuning range may be limited, and that efficiency will be a bit lower on 15 Meters (depending on both stray capacitance and stray inductance in the actual construction).

Denis,
Lots of static crashes, and very few signals.??
Dan, W2DLC

Step 1: Out with the DVM.
73 de ZL2DEX

I was tuning up on 160m, and the poor thing went dead. Won't power back up, either. Looks like I need to tear it down and find what mischief I did.

Finals? Or power supply? Suggestions from the symptoms?

73,
NG3P

Chris, what’s your antenna setup like? I am also on 40m CW, but in Ohio.

73

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