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henryrichter

Joined: Sun, Oct 14th 2018, 20:05 Roles: N/A Moderates: N/A

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connecting RF modules; impedance matching Jul 27th, 13:02 4 150 2 weeks, 3 days ago
broomstick antenna parameters Mar 13th 2019, 20:29 2 1,695 on 14/3/19

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connecting RF modules; impedance matching henryrichter 2 weeks, 3 days ago
This receiver will have three bands operating from 1.7-30 MHz. I am asking about switching bands in the LO. The LO consists of a Hartley oscillator (page 250 Secrets of RF Circuit design, J.Carr 3rd ed) three bandpass filters and a post-filter amplifier.
I plan on using pin diode switches at the output from the filters to the post-filter amplifier, using the two post filter diodes only from ("A Diode switched band-pass filter" D.DeMaw, QST January 1991).
My questions is where to insert the pin diode switch in the oscillator, where three bands requires three tank inductors. I believe I need to switch between the hot end of the tank and the gate. So, where?
between C4 and C1? C1 and R2, or R2 and the gate.
The alternative is to build three oscillators , each with their own inductor and then I simply need to switch the filters at their outputs to the post amplifier.
I actually prefer this route, because in a mock-up of the pin diode switch the power loss is considerable, so I have to be aware of that. I think that is the main purpose of my question: where to accept the loss?
It would be tight, but I can build three oscillators, in a way to put brass shielding between them.
As for noise I will do what is suggested in the 1991 article: when a switch is OFF, supply reverse voltage (+12V) to the diode cathodes and ground the anodes, to fully turn them off.
Any advice would be appreciated.
I see there are modern ICs available that act as a SPDT switch, but not a 3P3T switch that I need.
The three filters are 5 pole Butterworth band-pass filters. They require 50 ohm inputs and outputs. (rf-tools.com). The filter schematic has 50 ohm resistors at he inputs and outputs, which I think is figurative.
Just speaking of the filter input: the output of the oscillator is the secondary of the tank inductor. I can give the secondary an impedance of 50 ohms by either
1. winding correct turns on the coil to make the secondary 50 ohms all by itself, or
2, accept an arbitrary Impedance of the primary say 10 Mhz, in the center of my 5-14 Mhz band, and use N^2 to pick the number of turns in the secondary.
What is the best route?

connecting RF modules; impedance matching henryrichter on 27/7/20
I am building a superhet radio based on a 1994 publication, using Toko shielded transformers. This is my first radio project meant to educate as well as and function.
I have relied where possible on Experimental Methods of RF Design,the "book".
Each of these modules is in EMI protected die cast aluminum boxes:
1.Front end, consisting of tuned filter, BF244 RF amplifier and dual gate MOSFET Mixer ,40673.
2.Local oscillator
3.IF section
4.BFO/product detector.
My question is how to run the signal between the boxes:
LO to Mixer
Mixer to IF
IF to BFO
The options are to simply run a wire thru the boxes, to run a shielded wire, or to use 50 ohm BNC connectors and RG-59.

The latter is the question. As I understand it, using the 50 Ohm connector/ cable will require a step-down:step up toroid wound transformer with the proper turns ratio.
First, is the 50 ohm route necessary?
Second If so, am I correct in adding the toroids ?

The answer to this question will dictate the size of holes I drill in the boxes so the project is waiting for me to figure this out.

These are old parts with one page data sheets that don't include impedance. I would need to know the output impedance of the BF244 and 40673 transistors. The closest answer I can find online is 5000 ohms, all resistive to the GHz range.

A similar problem, the correct turns ratio, occurs if I insert a filter after the LO or on the Drain output of the mixer, before the first IF transformer.
The data sheet for a modern IF transformer, 42IF 101-RC,, similar in the older versions inductance and turns lists the primary impedance as 60K ohms. Thus to add a filter between the mixer drain and first IF transformer would require step down from 5000:50 , a 10:1 turns ratio, and set up from 50:60000 a 245:1 turns ratio!


The Toko coils are mounted on DB9 connectors for band changing. The construction is all " ugly style" on copper board.
Since I am drilling holes, should I include a "star ground" from the copper board and the variable capacitor to each box. Otherwise the electrical connection form the board to the box is the screwed on standoffs on the boards.


broomstick antenna parameters henryrichter on 13/3/19
Hello, I am seeking help with my first antenna build to go with a superhet radio I will be building.
The radio design is a 1991 book " Shortwave Superhet Receiver construction" by R.A. Penfold.
https://www.librarything.com/series/Babani+Electronics+Books (#276).
This design uses a MOSFET mixer and two IF stages. There are 9 possible add-ons: input filter and RF pre amp, product detector and BFO etc. This gives me plenty of room to experiment and learn.

I am awaiting parts- it took awhile but I've found everything, including the Toko RF transformers. In the meantime I settled on this antenna as being potentially more useful than a long wire, and not having to hire anyone to climb the palm trees at my home. The author calls it the broomstick antenna, a 1.7 m (6 ft) PVC pipe with a 30 cm capacitance hat at the top
http://www.hard-core-dx.com/nordicdx/antenna/special/bromstik.html

The radio will potentially tune 80-10M bands so I want the antenna to be able to do so also.
The author suggests dividing the coil wire into three sections: 1/3 spaced 2 wire diameters apart, 1/3 tightly wound, and 1/3 one wire diameter separation
Using 40 meters total length of 14 awg magnet wire ( 1.7 mm diam) and 33 mm diameter PVC pipe, each section requires 124 turns.
I calculate the inductance of the sections at 29, 85 and 44 uH respectively. The "Q" for each section is very high unless I have made a mistake: well over 600 depending on frequency.
This broomstick design is also from the 1990s. My questions are
1. I am using 40 meters of wire to have a half wave at 80 M. Is this reasonable?
2. The coils only use 120 cm of the 172 cm usable length of the long pole. I assume I should place them next to the top next to the "cap", and not in the middle of the pole.
3. Understanding this is all experimental, would a design with 2 or 4 sections of wire spacing make more sense as a first try?
4. The author mentions having the antenna as close to the receiver as possible. I want it outside on my porch, approx 6m away. I can insert a ground rod close by. Should I use coax for the antenna lead?
5. The author mentions using an antenna tuner within 1m of the antenna. In other words at the base of the antenna. First the radio will have an input filter and RF amplifier before the MOSFET mixer and first IF stage. Doesn't the "input filter" which is an LC resonator built with a variable capacitor and a Toko RF transformer serve the same function as an antenna tuner, while also including the lead in wire?

I was a ham operator long ago at 11 years old, WN2TBG. I had a Hallicrafters receiver and a borrowed 2M transceiver. The hobby was fun but didn't survive the distractions of high school. Recently I started building radios, crystal sets and a regen receiver kit. They work, sort of. Part of the problem perhaps, is that much of my house is plaster and metal lathe which might act like a Faraday cage blocking signals? At any rate the regen kit works best outside, which is why I want to move my antenna outside.
I am progressing thru levels of complexity, and will eventually build something that really works.

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