27 MHz Amplification Query

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Thanks a lot.......... and really sorry for disturbing you when you yourself are busy in your project! I will build the circuit and come back with the results soon!
 

No need to apologize, it's me that's slow to respond. I'm staying in a small room for a few weeks while rebuilding my home/lab so I have no facilities other than a slow laptop to work with at the moment. I've had to demolish the old buildng and clear the site ready for the new construction starting tomorrow so most of my time has been spent working with a shovel, wheelbarrow and pickaxe (not the electronic version!)

Brian.
 

I have ordered for the parts for the receiver pre-amplifier and in the meanwhile was figuring out the design of tuned amplifiers but i have some following questions:
1. How do i calculate the output impedance so as to select appropriate load? do i need to use Smith Chart for that?
2. I tried to devise a cascaded arrangement of driver, preamplifier and power amplifier and then simulated it in a java applet but it seems like the waveform is quite distorted..... is it because of non-linearity of class C amplifier or poor design or i am using wrong simulator and should go for multisim etc?

 

1. you can use a Smith Chart but it won't give you actual component values to use. The best method would be to search the net for "pi networks" which are a simple circuit using two capacitors and an inductor which resonates with different input and output impedances.

2. With no bias on the transistors you will get significant distortion. However, it may not be important as all the unwanted frequencies will be multiples of the original 27MHz and should be filtered out by the antenna matching network.

In general, for AM transmitters with analog modulation you should use linear amplifier stages with low distortion, OOK transmitters like yours are AM but the modulation is 100% square wave anyway so linearity isn't important. In FM transmitters it is normal to the output stage to be hard driven with high distortion because the modulation is carried by shifting the frequency not amplitude.

Brian.
 

I have another two queries:-
1. I have read in a book that the antenna impedance having half wave straight dipole and reflector to be around 36ohms but the one with high impedance folded dipole results in an antenna resistance with parasitic elements can be about 150 to 300ohms. My question is what might be the approximate feed point impedance of a 4 element Yagi antenna (which may include a reflector, high impedance dipole and 2 directors)?

2. I used the Band Pass Pi-network and it seems like the output waveform has become more stable and smooth a/c to the simulation.

Can you please verify this and give suggestions for improvements. I have made the output impedance of the pi-network at 300ohms b/c i thought that there is a possibility that the feed point impedance might be around it for the 4-element Yagi Antenna. I will recalculate if its not!

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Q1 is selected to be 2N2222A; Q2 is 2N2219 and Q3 is 2N3866.
 

1. generally, as you add more elements the impedance at the center feed point of the dipole increases. The amount it increases gets smaller as each new element is added so the ones nearest the dipole have most effect. The trick is to reduce the imedance by 'folding' the dipole. You will see it used on almost all TV/VHF antennas where the dipole is like a squashed loop. The length from end to end of the dipole is still the same but the far ends are joined to form a continuous length of conductor. Note that doing this makes the dipole look like a short circuit to DC so be careful to ensure there is a capacitor in series with it somewhere to prevent it shorting your power rails.

The impedance you use isn't particularly important as long as you match the transmitter to the antenna but the cable you use to join them has to be matched as well. This is why 75 Ohms is commonly used. There is nothing magic about 75 Ohms but a standard somewhere in that region had to be set so that cable manufacturers could make compatible products. You would normally make the transmitter output impedance 75 Ohms and use a 75 Ohm coaxial cable to connect it to 75 Ohm antenna.

2. The simulation looks OK but try something else: see if you can make it resonant instead of bandpass.
In a pi-matching network the idea is that you use a series inductor and a capacitor at each end of it to ground (so it looks like the Greek symbol pi). To maintain resonance you can increase one capacitor and reduce the other as long as their combined capacitance still tunes the inductor to the right frequency. This means there are infinite combinations of values that maintain resonance, effectively they are in series with each other and in parallel across the inductor. The relative capacitor values will determine the input and output impedances of the network so by picking appropriate values you can match the best impedance to transfer power from the amplifier to the best impedance to match the cable/antenna. In most applications you will find that both are made variable so they can be adjusted for maximum power output.

Brian.

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1. generally, as you add more elements the impedance at the center feed point of the dipole increases. The amount it increases gets smaller as each new element is added so the ones nearest the dipole have most effect. The trick is to reduce the imedance by 'folding' the dipole. You will see it used on almost all TV/VHF antennas where the dipole is like a squashed loop. The length from end to end of the dipole is still the same but the far ends are joined to form a continuous length of conductor. Note that doing this makes the dipole look like a short circuit to DC so be careful to ensure there is a capacitor in series with it somewhere to prevent it shorting your power rails.

The impedance you use isn't particularly important as long as you match the transmitter to the antenna but the cable you use to join them has to be matched as well. This is why 75 Ohms is commonly used. There is nothing magic about 75 Ohms but a standard somewhere in that region had to be set so that cable manufacturers could make compatible products. You would normally make the transmitter output impedance 75 Ohms and use a 75 Ohm coaxial cable to connect it to 75 Ohm antenna.

2. The simulation looks OK but try something else: see if you can make it resonant instead of bandpass.
In a pi-matching network the idea is that you use a series inductor and a capacitor at each end of it to ground (so it looks like the Greek symbol pi). To maintain resonance you can increase one capacitor and reduce the other as long as their combined capacitance still tunes the inductor to the right frequency. This means there are infinite combinations of values that maintain resonance, effectively they are in series with each other and in parallel across the inductor. The relative capacitor values will determine the input and output impedances of the network so by picking appropriate values you can match the best impedance to transfer power from the amplifier to the best impedance to match the cable/antenna. In most applications you will find that both are made variable so they can be adjusted for maximum power output.

Brian.
 

Sir I asked you about how to know the impedance of these Yagi-antenna b/c without knowing it how would i match its impedance with that of the cable and the transmitter......

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All I was able to find that by decreasing the spacing b/w the parasitic elements you get to decrease the impedance but whta would be its value?? It was stated that for this specific set-up of Yagi-Uda design employing high impedance folded dipole it is around 150-300ohms.....

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I also read that for a folded dipole antenna resistance is equal to 72 multiplied by the square of the ratio of the total diameter of all conductor sections to the diameter of the open section...... but by adding parasitic elements i think it gets decreased so what would be that value? how do i calculate it??

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I also came across a chart but it was for straight dipole which had values for no. of elements and in parallel rows their corresponding gain, impedance etc
 


Hi Brian

You have that around the wrong way mate

a straight dipole has a feedpoint of 75 +- a few ohms
adding reflector and directors lowers the feed impedance down to anything between ~10 and ~ 40 Ohms
and hence we use gamma, delta matches to match to 50Ohm coax

A folded dipole has a feed impedance of 300 Ohms and the addition of elements will again lower that
to ~ 200 Ohms ( give or take a few) and that why we can use a 4 : 1 Balun to match to 50 Ohm coax
And for a receive only system such as TV, they don't really worry too much about the mismatch when using
75 Ohm coax, a 4:1 balun to a ~ 200 Ohm feedpoint ... its close enough

That info is readily available in RSGB, ARRL handbooks and on the www

Dave
 
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Oops! sorry - I don't have access to all my data books and manuals at the moment, everything is in storage units while I move house. I'm working from memory which is obviously not as sharp as it used to be!

Regardless, the main problem RMMK will have is connecting the transmitter to the Yagi which at 27MHz is going to be quite large. It's the connecting cable that will cause the problem and need to be matched at both ends.

Regarding antenna impedances, the calculation is not simple, especially if matching components such as a balun are also needed. I suggest downloading antenna design software to make it easier.

Brian.
 

I am thinking of using a flat twin lead transmission line having characteristic impedance of 300ohms with per foot capacitance of 6pF.
With the pi-network i have the transmitter at output impedance of 300ohms, This way probably the transmission line and the transmitter have matched impedance. For the Antenna I am considering a 4 element Yagi-Uda Antenna:-
**broken link removed**
The folded dipole resistance is 648ohms........ But i am not sure what will be the feed point impedance when this dipole gets incorporated into the 4-element Yagi arrangement....... will it be 300ohms? Plz help....

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Also this impedance for flat twin lead was written in a book! I came across such a line in the market but the problem is I am still sceptical (it maybe not have 300ohms?)...... How can I check the characteristic impedance of that line b/c i couldnt find any writing on the cable which was available in the market!
 

betwixt said:
Oops! sorry - I don't have access to all my data books and manuals at the moment, everything is in storage units while I move house. I'm working from memory which is obviously not as sharp as it used to be!
Click to expand...

Haha yeah I know the feeling

Regardless, the main problem RMMK will have is connecting the transmitter to the Yagi which at 27MHz is going to be quite large. It's the connecting cable that will cause the problem and need to be matched at both ends.
Click to expand...

Yup and he has just said in his next post he wants a 4 ele Yagi ... for 27MHz its going to be HUGE. ( a 3 ele will give 6dBd gain, a 4 ele maybe another 0.5 dB)
Stay with a 3 ele at the most it will still be a lot for you to handle


Maybe now you might get the idea why we were encouraging you ( a way back near the start of the thread) to look ay much higher freq's. High gain antennas are much, much smaller and much easier to manage

You would be wise to just play with a dipole ... folded or straight for a start.
If you use a straight dipole, you can match 75 Ohm coax straight to it without any problems

Dave

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RMMK said:
The folded dipole resistance is 648ohms........ But i am not sure what will be the feed point impedance when this dipole gets incorporated into the 4-element Yagi arrangement....... will it be 300ohms? Plz help....
Click to expand...


No, as I have already said, a folded dipole has a 300 Ohm feedpoint


Dave
 

yes

because in figure a, they are playing with a 3 wire (rod) element sections
and in figure b they are playing with uneven diameter element sections
either one of these variations from the standard, significantly changes the impedance

instead of the standard folded dipole as seen at the bottom of the first column

it helps if you read and understand the descriptions given :grin:

cheers
Dave
 

then sir do u agree with my previous post that this high impedance dipole provides some 648ohms rather than 300ohms?? Also how much will this decrease if its on the 4-element or ko 3-element Yagi? I cant find the approximate value of impedance so that i may match it with the cable and the transmitter!!
 

but you DONT want to use that style .... there's no point to it

if you want to use 300 Ohm ribbon ( ladderline) just use the standard folded dipole
or use a straight dipole and use 75 Ohm coax


dave
 

but sir the parasitic elements will provide me more gain and a better front to back ratio of around 15db...... I was also provided with a solution in this thread for helix antenna and it was stated by Brian that in normal mode their size is smaller while in axial mode its comparable to its wavelength!
I was only able to find this calculator and its giving me horribly large value of length of wire!
https://www.daycounter.com/Calculators/Helical-Antenna-Design-Calculator.phtml

Also what would be the gain of folded dipole antenna only?
 
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to get 15 dB gain you will need around 18 elements, tho it is possible to build a yagi that big on 27 MHz it is pretty impractical
for a start, the boom is going to be ..... well I found a design for a 5 element yagi for 27MHz and it's 7.7m long .... so multiply
that by 3 and add another metre or 2 for the 3 additional elements .... your boom is going to be ~ 25 metres long ( ~ 80 ft)
and around 5.5 metres (~ 18 ft ) wide

Do you understand how huge this is ?
The huge, set in concrete foundation, tower you would need to support an antenna this big ?

I used to work the world on 27MHz with 10 - 15W and a half wave dipole


Dave

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those are not parasitic elements in that image you posted. its just 2 variations of a folded dipole. ( its not a yagi)

On the HF ( High Frequency) bands gain is easier to achieve with amplifiers because of the big size of antennas makes then impractical
On VHF and up into the microwave bands, high gain is easier to achieve with multi element yagis, dish antennas etc


Helix antennas are again more suited for the UHF bands say ~ 400 - 2000 MHz where you can achieve reasonable gain

Its all a compromise :grin:
I have been building antennas for 30 + years for frequencies from 7 MHz to 24 GHz.
And tho I am no expert, I have a reasonable grasp on the issues associated with doing such constructions ;-)


cheers

Dave
 
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    RMMK

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Thanks a lot sir! I really appreciate your comments! They really help! Sir currently i am using a whip antenna sort of arrangement! Its a piece of 13 inches long wire having a dia. of 0.027inches. I have placed an inductor of 10uH. The values were found using this calculator:-

https://www.daycounter.com/Calculators/Whip-Antenna-Design-Calculator.phtml

But its working not much....... I dont know the gain if any it provides and is only providing me a pathetic range of 3-4m. This range really made me ditch it for the application i was thinking it to employ! Now side by side I am going to work on RF modules 433Mhz ones. I am thinking of placing the same modifications as above i.e., 4-element Yagi (it would be much smaller), the power amplifier on the transmitter (that cascaded arrangement with a power amplifier stage of 2N3866) and the preamplifier (that BF998 version i uploaded before) on receiver side. Ofcourse I will change the values of the circuit to appropriate ones for 433MHz.

My questions are:-
1. Will the above mods i am thinking will work for 433MHz? Seriously...... 8-O
2. Is there a way to reduce the size of the yagi-antenna for 27MHz version just like the one is done for in the whip antenna since its smaller that 1/4 wavelength and tuned to the particular frequency using the capacitance of the whip and the inductance of inductor.....??

I know Sir these both questions feel naive......

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sir i was referring to directors and reflectors when i called for parasitic elements!
 
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its all in getting the best performance out of your 27MHz transmitter circuit. Over the last few pages as you have worked with Brian, you will be starting to realise that working with RF signals is very different to working with audio. Circuit design and layout on a PCB becomes important if you want things to work properly.
You cannot just solder a bunch of wires and components together in a rough way and expect it to work like you can with an audio circuit.
Keep working at it and Brian and I will help where we can personally I am not a designer ... never have been. I can tell you the basic operation of a circuit, but not the deep electronic theory.
Primarily, I am a constructor. Give me the parts and a schematic and I will build it for you ... sort of thing
I also enjoy modifying existing circuits/systems so they meet my requirements ... but that is another whole other story haha

Back to your comments .....

on a good designed and constructed circuit for 27 MHz, 0.5W and a 1 metre whip antenna you can easily get several km's around your local area
from a hilltop you would easily get 5 - 10 km's
A 2N3866 should give you ~ 1 - 2 Watts on 27 MHz ( ~ 0.5 W on 433 MHz).

Let me see if I can dig up any straight forward 27 MHz TX and RX circuits for you. A well designed and laid out RX circuit is just as important to be able to receive and amplify a weak signal

Using 433 MHz allows for both compact transmitters, receivers and antennas



sir i was referring to directors and reflectors when i called for parasitic elements!
Click to expand...

Ahhh, yes that's correct. I though you were referring to that image you posted with the folded dipole variations


Ohhh and you don't need to call me sir .... thanks for the respect, but I have not been knighted by a king or queen

cheers
Dave
 

davenn said:
Ohhh and you don't need to call me sir .... thanks for the respect, but I have not been knighted by a king or queen
Click to expand...

Its just that even though i am a civilian, i study in a prestigious military university in my country. In the university we are required to call every person who is senior to us by "sir"....... I picked that habit from there! Its in-built now

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Sir (again, i cant help it..... if i somehow loose this habit i will get a fine of around US$10- equivalent in my currency) are u implying that i will be able to get range in kilometers with the modifications i am considering?
On the circuit construction; please give me the tips for building them. I also uploaded a pic of my construction before in this thread and I must mention that sir Brian actually accepted it by saying "it should be okay".......
 

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