VHF Low Pass filter for DRA818V module

 It is best to use the DRA818V VHF voice transceiver module with a low pass filter.

Since I'm building a VHF/UHF transceiver using the 818V and 818U modules, the VHF low pass filter was missing from the build.

 

This design was based from the one here and only some small values changes made , I placed adjustable cap's on the center elements (the 56pF), so used 10-60pF trimmer cap and used 32pF on the ends (10+22pF parallel). 

Schematic:

 After build did some measurement on the spectrum analyzer:

Final iteration of the trimmers:

 Attenuation at 200Mhz:

Filter shape from 100 to 300Mhz without normalization:

...now I need to finish the remaining of the transceiver..

 

Have a great day!

FM crystal radio

I'm back!...Along with the lousy photos and the rusty English...

Having already build a lot of receivers with different approaches I have never tried FM with a "crystal" radio. Now is the time.

This is a simple circuit and honestly I wasn't expecting much performance but is fun and helps getting some momentum for future builds.

I can only get two very faint stations and with an external amp based on an LM386, my "crystal" earpiece I guess is not crystal....

My implementation



The schematic was "borrow" from here. Go on, see the original and nice implementations on that site.

Original schematic from the author site

L: 5 Turns of 1mm silver wire on an "Edding 3000" pen (around 12mm), diode connects to 2.5 turn (half the coil), spacing is around 2mm
D: OA90 or 1N34 (not sure, it wasn't marked) I tried several glass diodes and results were similar.
R: 150 K
C3: 18p
C1: 82p
C2: used one trimmer of 25p (80pf on the original schematic)
Antenna was around 25cm of wire and the 2 station's received are less than 8 km from the shack.

Have fun.

New shack + New VHF TX

I'm currently making some tests with the 21.4Mhz filters bought in last ham fair for an Si570 based VHF FM transmitter. Tests are promising and looking good so probably a final schematic in the next few days.
Here's some images on progress so far:
The original schematic, bellow.
Right now after some changes the tuned circuit on the 10.7Mhz oscillator/ FM modulator (21.4Mhz) is now on a red toroid. The output of the ne602 has an output coil of 6 turns on the NE602 side (pin 4 and 5) and 1 turn for antenna matching (amplifier strip later) hopping to give 1500/50 impedance conversion...or near :)
21.4Mhz filter is connected to the NE602 via a 10nF cap, so does pin 3 to ground. The 100K on the oscillator collector was removed. A small cap was added from the top the tuned circuit to the ground, there's a 56pF cap in parallel with the ferrite ring, resonance viewed by the dip meter is around 22Mhz, the red variable cap should put it on 21.4 after tuning.
Even with the 21.4 filter and the output coil I still have small harmonic leakage on the Si570 frequency - 21.4 * 2, that is: the frequency I hope to transmit (Si570 frequency - 21.4Mhz) minus 21.4Mhz, or Si570 frequency - 4 * 10.7Mhz :) Did not make yet tests on other harmonics. Si570 oscilation frequency is on the high side for that matter.


Mean while I arranged a secondary shack at my parents house, they were kind enough to prepare some extra electricity plastic tubing, when the house was built, from the shack to the rooft. That way it's easier to install antenna cabling. Unfortunately not all weekends I pay them a visit but it's nice to know that I am radio ready there case needed. At the moment I only have an VHF antenna tuned for the marine band but I left there a balun and some wire ready to deploy on a simple dipole form.
here's the setup:


Later on will get a wood rack on top of the FT-102.

Antenna experiments - coffee caps

First let me tell you I am not affiliated in any way with the coffee capsules brand reported here (Nespresso). Neither I like closed format or standards.... from what I can tell this experiment could had been realized with the use of any other brand of coffee (Nexpresso is a proprietary format....whatever!)!

Basically I wanted to see the influence of capacitive elements in antenna impedance values. The antenna is just a vertical element of around 50cm (1/4 wave on 144Mhz band) and no ground plane was made except that the meter body acts as such (more or less).
Here the 2 versions with and without the capsules:


Capsules were randomly distribute over the element and are made of aluminum (were empty since I drink the coffee)....

Now the obtained values with the MFJ analyzer:



...so the "scientific" fact is that coffee capsules can lower your antenna SWR.

Have fun!

p.s. I promises one of this days I will make a more mathematical analysis on this subject but for now this is amateur radio...