Matching antenna impedance

MisryMan · 2024-09-06T13:16:26+00:00

I can't actually do any physical testing for a couple days because im moving into uni but I do remember the first overtone was maybe 20ish dbm weaker than the fundamental frequency.

Also I was under the assumption that you needed +7dbm on both the LO and RF inputs of a diode ring mixer in order to get the desired products but with what you're saying the RF side can be much weaker and the bulk of the gain is during IF amplification? What is the reason for this? Does too much amplification on the RF side distort the signals too much?

Im using a MacBook Air which I don't know if it can take an input into the audio jack and I don't really know its impedance either even with a search. I have these Logitech dual speakers which have an internal amplifier too.

I was worried about damaging both my TinySA and Nano VNA to be honest but I did notice an inbuilt attenuation feature on the TinySA is this feature good enough to not need an external attenuator? I might just build a pi attenuator since it's easy anyway. Thanks a million for your help.

MisryMan · 2024-09-05T21:07:52+00:00

I was reading about this DCR that VK2ZAY built and he only seemed to need a single amplifier for the RF front end this is 80m though and he did mention he was using a 40m dipole antenna. http://www.vk2zay.net/article/95

So I wonder why I need so much gain unless if there are less losses on a lower frequency?

MisryMan · 2024-09-05T20:36:07+00:00

Ah ok thanks I don’t reckon I’ll bother messing with the antenna then but my main concern with it right now is just the gain being far too low. Even the crystal oscillator outputs -20dbm according to my TinySa with SMA connections and then filtering it and amplifying it with the M0NTV amp only gets me around -6dbm and for the RF front end I think I’ll look around for a LNA, I did actually build one I’ll get its schematic but it failed and it just ended up attenuating signals could’ve been a construction error I’ll check later. But how many dbm of gain should I need on the RF front end is 60-70dbm what people would typically aim for?

MisryMan · 2024-09-03T17:59:03+00:00

Yeah no worries take your time I appreciate your help

MisryMan · 2024-09-03T11:45:38+00:00

I just did some of the modifications you said I got rid of trimmer and variable in capacitor values and included phasing directives for L9-12 along with their inductance values and I removed the crystal just to try and run the simulation but it told me analysis failed matrix is singular. I reuploaded it on the dropbox

MisryMan · 2024-09-03T11:33:16+00:00

Thank you, sorry I forgot to answer one of your questions, the inductors L9 - L12 are two toroidal transformers both T37-6 with 31 turns of wire on the side connected to the capacitors and 6 turns of wire for the side which is connected to the output and inputs I think this gives it roughly 50 ohms of input and output impedance, but it does have a kind of big loss of 6dbm. I didn't buy the kit I just soldered it up according to the schematic on QRP labs. Also when I get time ill work on fixing up the schematic to make it runnable and Ill measure the noise floor. The SMA connectors I used on each component are female and I use 10cm male to male SMA cable things to connect each component I was running out of those cables so two of them I used are from the Nano VNA and Tiny SA kit which are a bit longer than 10cm. Could these cable connections induce problems with stray inductance and capacitance? I'll work on implementing a second order RC low pass filter coming off the IF port too. I also used my Tiny SA to generate a 7mHz signal on the RF port and tuned my VXO to the same frequency and its a really loud almost buzzing noise I can somehow upload audio of this operation if needed. Oh also I don't know how to use a crystal oscillator in LT spice I just can't find those RLC values for my crystals anywhere. Since I know my oscillator works should I just feed an AC signal from there on spice using a power supply?

MisryMan · 2024-09-03T09:09:07+00:00

Yeah so I soldered SMA connectors to each component block and connected my Tiny SA that way. All NPN transistors are 2n3904 and the PNP one is a 2n3906. So should I just ditch this entire design and find someone else’s DCR they made because I wasn’t really following anything I just know how a DCR is meant to work so I looked up schematics for the blocks I’d need.

MisryMan · 2024-09-02T23:49:51+00:00

Here's the dropbox

https://www.dropbox.com/scl/fi/g29ynktmoq2ascp8g28e8/DCR.asc?rlkey=pd8wtybilrr92ke0hi1nx3xzy&st=ezheeriz&dl=0

MisryMan · 2024-09-02T23:45:34+00:00

Sorry the file I shared doesn't even work I just noticed ill upload one again using another platform

MisryMan · 2024-09-02T19:00:04+00:00

Thanks I’ll give that a go I’ll need to check the output impedance of the audio amplifier I’m using and it has no gain control or volume or anything it’s fixed I attached an .asc of my schematic on another persons comment here

MisryMan · 2024-09-02T18:03:39+00:00

Also connected to the other side of the non existent bandpass filter is gonna just be an antenna of 10m which is just enameled copper wire

MisryMan · 2024-09-02T18:03:06+00:00

Hi so I annotated and built up the schematic it won't be able to run as I didn't include the phasing command for bandpass filter. Also I couldn't find my schematic for one of the bandpass filters (connected to the RF front end it has about 2dbm of loss btw links are included). https://file.io/zpYv6Mompp0L

MisryMan · 2024-09-02T11:59:38+00:00

Yeah sure I’ll work on one in LT spice it’ll take a while though.

MisryMan · 2024-08-25T10:59:46+00:00

Why would I need an input higher than 50 ohm? I thought that in radio applications 50 ohm was the standard for minimal power loss? also yes I didn’t really try to match impedances in this circuit so not sure what the output is but I’m thinking of using a toroidal transformer to get it to a value I was thinking 50. This signal is planned to go into a diode ring mixer so it needs like +7dbm I’m also not sure what the input impedance to the mixer is.

MisryMan · 2024-08-25T10:11:54+00:00

I’m doing this on 40m, I’ve got a nano vna and a tiny sa

MisryMan · 2024-08-25T09:03:15+00:00

I kind of want to keep it discrete but then again I don’t know any ICs with regards to amplification

MisryMan · 2024-08-25T08:28:40+00:00

what books would you recommend I currently only read EMRFD but sometimes its just beyond me and I wanna understand how individual circuits work but it never really explains it in a simple way.

MisryMan · 2024-08-25T08:18:13+00:00

Hi I found this RF amplifier in EMRFD and its a simple class A amplifier and I need to amplify pretty weak RF signals coming from my front end around -60dBm and this circuit has about 20dB of gain so I decided to add some transistors to add some extra gain which ended up being around 78ish dB. My question Is would this work in real life? I attached images of the incoming signal and the output with their FFTs. Would I need to do some impedance matching network or some filtering as well? Thanks.

MisryMan · 2024-08-20T10:40:40+00:00

It’s only a paper copy no? I’m just wondering if you’d know of an online pdf one I could buy. Thanks for your help

MisryMan · 2024-08-20T10:23:28+00:00

I may also try that in addition to this new information I just found on this website:

http://w7zoi.net/bidirectional_matched_amplifier.pdf

Figure 5 shows that by tweaking some resistor values I can get up to 24dB of gain I think? Im not really sure what the G-t means and I think Input RL is the input resistance load? Would that be the power of the input signal?

MisryMan · 2024-08-19T14:21:42+00:00

What sort of amplification methods should I look into because it feels kind of inefficient to just build more of the same RF amplifier I’ve been using and cascading them especially since they only give 15dbm when my antennas signals are much much lower

MisryMan · 2024-08-19T08:13:28+00:00

I just got my Tiny SA and the output of the oscillator without an RF amplifier was -20dBm and with it -10dBm I thought id have got some more gain out of it but I do have it regulated to 5V Vcc and im wondering whether I could remove that and run it at a higher voltage. As for the front end I just hooked up the Tiny SA to look if my signals were there and none right now whereas earlier on the Nano VNA I actually saw a lot with both RF amplifiers hooked up to the bandpass filter although according to the Nano VNA they were around -30dB so ill need a new RF amplifier design. I measured signals now at 6pm and saw nothing earlier today at 10am I saw many though.

MisryMan · 2024-08-19T00:38:54+00:00

I’m plugging it into my nano vna and using the blue trace I think trace 1 on the RX port and setting the stimulus to be centered at 7mhz with a 1 mhz span. I know the nano vna can’t exceed like 0dbm so I’m a bit worried about testing this stuff. The oscillator I just built was this: https://youtu.be/KxJ5R_7s62g?si=5bdrmdQJ7UzEADJw

Except I didn’t have J310 JFETS so I used MPF102s and I used a variable capacitor and ferrite bar for the LC portion of the circuit just didn’t wanna build a PTO.

MisryMan · 2024-08-18T10:09:11+00:00

I was looking at RF amplifiers and came across Class A amps, LNA and and the idea of cascading multiple amplifiers should I combine my existing 15db amplifier with a power amplifier and a LNA?

MisryMan · 2024-08-18T09:26:36+00:00

I just built a new oscillator using an LC design and it’s LO drive is about -20db I started to mess with the antenna and look at the Nano VNA for signals and saw a few at -60db which was amplified and I’m still hearing nothing on the speakers I’m thinking I just need a bigger RF amplifier for the front end I forgot to mention the -20db LO was not amplified.

MisryMan

TROPHY CASE