Why Yagi antenna has a loop?

Joined
Jun 2, 2024
Messages
59
I need to know why a Yagi antenna used to receive UHF has a driven element like a loop?

Usually, a Yagi antenna should have a dipole type driven element to receive the signals. But connecting the two wires to the loop would short the wires causing very low impedance. How is that going to work?

Yagi Antenna.jpg
 
Joined
Jun 2, 2024
Messages
59
It does have a dipole driven element, a folded dipole.

Here's a page with some good photos of how the folded dipole is constructed: Yagi 6El 436MHz

Some folded dipole theory: Folded Dipole
I got this one but the dipole is closed like shorting two wires. How it will work?

This is just like stripping the core wire of a coxial cable and making a loop out of the copper wire.
 

paulears

Member
Joined
Oct 14, 2015
Messages
846
Location
Lowestoft - UK
No it's not. It IS a short - but only to Direct Current, as on a meter. To RF, it is not a short at all. In fact, while it is 0 Ohms resistance it is 300 Ohms impedance. This means a little matching network is fitted inside to reduce it to 50 Ohms, often called a balun - balanced to unbalanced transformer.
 
Joined
Jun 2, 2024
Messages
59
No it's not. It IS a short - but only to Direct Current, as on a meter. To RF, it is not a short at all. In fact, while it is 0 Ohms resistance it is 300 Ohms impedance. This means a little matching network is fitted inside to reduce it to 50 Ohms, often called a balun - balanced to unbalanced transformer.
But I believe that there should be a gap between the folded ends because a simple dipole antenna works with two independent arms, and even if they are folded, they need to be two independent arms.
 

AK9R

Lead Wiki Manager and almost an Awesome Moderator
Super Moderator
Joined
Jul 18, 2004
Messages
9,675
Location
Central Indiana
Folded dipoles are very common in VHF/UHF antenna designs. They consist of a loop, which is sometimes squashed to form an oval, which has a gap. The feedpoint is at the gap. One side of the feedline connects on one side of the gap while the other side of the feedline connects to the other side of the gap. While this might appear as a short circuit to direct current, it is not a short circuit to alternating current such as radio frequency energy. The size of the loop is tuned to the desired operating frequency.
 
Joined
Jun 2, 2024
Messages
59
Folded dipoles are very common in VHF/UHF antenna designs. They consist of a loop, which is sometimes squashed to form an oval, which has a gap. The feedpoint is at the gap. One side of the feedline connects on one side of the gap while the other side of the feedline connects to the other side of the gap. While this might appear as a short circuit to direct current, it is not a short circuit to alternating current such as radio frequency energy. The size of the loop is tuned to the desired operating frequency.
Now I got it. So, it's like a loop antenna used in the rabbit ears indoor antenna?

But the folded loop can touch the metal rod below it or not?
 

AK9R

Lead Wiki Manager and almost an Awesome Moderator
Super Moderator
Joined
Jul 18, 2004
Messages
9,675
Location
Central Indiana
But the folded loop can touch the metal rod below it or not?
Do you mean the part of the antenna that runs perpendicular to the elements? That's the boom. It needs to be insulated from the driven element, but not necessarily the reflector or directors.
 

Echo4Thirty

Active Member
Premium Subscriber
Joined
Oct 6, 2021
Messages
758
Location
Spring,TX
The fact that its shorted to ground at DC is an advantage for lightning and static discharge dissapation. At its proper operating frequency it is not a "short" so to speak.

No different than a tapped coil design on gain mobile antennas. If you measure them with an ohmmeter, they are zero (or very low) ohms, but at the operating frequency it presents a 50 ohm load to the transmitter.
 
Joined
Jun 2, 2024
Messages
59
Do you mean the part of the antenna that runs perpendicular to the elements? That's the boom. It needs to be insulated from the driven element, but not necessarily the reflector or directors.
Yes, I am talking about the driven element. Some people attach it directly to the boom without using any insulator.
 
Joined
Jun 2, 2024
Messages
59
The fact that its shorted to ground at DC is an advantage for lightning and static discharge dissapation. At its proper operating frequency it is not a "short" so to speak.

No different than a tapped coil design on gain mobile antennas. If you measure them with an ohmmeter, they are zero (or very low) ohms, but at the operating frequency it presents a 50 ohm load to the transmitter.
Okay, I need to clarify one thing about the impedance. I know that impedance is resistance (ohms) of any conductor. So, in any circuit, there are a input and output impedance. The input impedance is the resistance of the input connectors (of the load) to where the signal/electricity flows from the output source. The output impedance is the impedance of the output connectors (of the source) from where the signal/electricity flows to the input load.

This concept is similar to an audio amplifier when connecting with a speaker. As per the rule the input impedance should always be higher than the output. So, an amp with 4 ohms can be connected with a speaker of 4 ohm or higher.

Similarly, any antenna is an output to the receiver. So, the impedance of the antenna should be less than the receiver's input connectors. May be because of this reason, a matching transformer is required.

But my question is, how to find the impedance of the receiver's input?

If I connect a multimeter to the two output connectors of an antenna, I will get the output impedance, but if I do the same with the input connectors of a receiver, then am I going to get the output impedance?
 

paulears

Member
Joined
Oct 14, 2015
Messages
846
Location
Lowestoft - UK
You really have got this wrong. Resistance and impedance are VERY different. practically everything you may know about audio amps does not relate to RF. Your meter does NOT read impedance. You really need to stop making statements that frankly, are rubbish.

By the way, if somebody has bonded the dipole to the boom, they are an idiot.

Think about what you know incorrectly, and ask yourself how the RF gets from transmitter antenna to the receiver? there are not even any wires! Also, impedance is linked to AC and frequency, so your speaker analogy fails even worse because your amp might well have 4 Ohm impedance at 1KHz, but different at 50Hz or 20KHz.
 

kf8yk

Member
Joined
May 3, 2003
Messages
735
I know that impedance is resistance (ohms) of any conductor.

Only in the special case of a purely resistive circuit, which is generally not the case when dealing with antennas. While the unit of resistance and impedance are both the Ohm, they are not the same.

As per the rule the input impedance should always be higher than the output.

No such rule in the radio frequency world, in fact to maximize power transfer the input and output impedances should be equal.

If I connect a multimeter to the two output connectors of an antenna, I will get the output impedance

Nope, a multimeter is not the right tool for radio frequency testing. An impedance meter or antenna analyzer would be a better choice.

I would suggest picking up a copy of the ARRL Handbook at the library & reading the chapters on electrical & AC theory, trying to use DC principles with antennas is not going to work.
 

paulears

Member
Joined
Oct 14, 2015
Messages
846
Location
Lowestoft - UK
I'm really not trying to critical here, but you've shown some really elementary misunderstanding but absolute certainty that you're right? Not your fault you've not given the physics (and maths) a go - but RF is not something obvious, or even sensible.

Hopefully you will be Googling, but also consider what is called VSWR. If you have a 10W transmitter, with a length of 50 Ohm cable, and an antenna with 50 Ohm impedance, then if the antenna is tuned (as in it is resonant) at the frequency of your transmitter, then (if we ignore the bit of power that is warming up the cable) all that 10W is radiated into what is often called 'free space'. If you have a VSWR meter, or a modern electronic devices such as a VSWR capable analyser, it will show 1:1 as a reading. A perfect match. If you replace the antenna with a dummy load that doesn't radiate at all, but converts your 10W into heat - that too is a 1:1, even though nothing escapes from it. Oddly - the dummy load is actually a simple resistor - 50 Ohms. The other thing you'll discover is that if you analyse an antenna, simply putting your hand near it changes the results. Not mathematics and calculator stuff - simply waving your hand can change the readings. Even more oddly, if your transmitter is presented with a poorer load than it is happy with, it can be damaged - your 10W going one way can be reflected back up the coax cable. Your transmitter will probably notice it and throttle back, but some don't, and then overheat and die!

You can use 75 Ohm impedance cable on a receiver and it works pretty well, and is often cheaper, but you can't do it with a transmitter. The matching matters.

If you want to be even more confused, Google Alford Slot antennas. Usually found in microwave systems, this is a piece of vertical tube. A slot is milled on one side, and the coax applied half way up the slot - one connection to one side, one to the other - and it radiates HORIZONTALLY polarised, all round! A metal tube with 0 resistance. If you can understand how these work you are better than me. The maths is very complicated, but they work great!

That's enough to Google for now. See if you can get a grip on this stuff and hopefully it will begin to make sense. A resistance meter is pretty pointless with RF.
 

prcguy

Member
Premium Subscriber
Joined
Jun 30, 2006
Messages
15,799
Location
So Cal - Richardson, TX - Tewksbury, MA
Okay, I need to clarify one thing about the impedance. I know that impedance is resistance (ohms) of any conductor. So, in any circuit, there are a input and output impedance. The input impedance is the resistance of the input connectors (of the load) to where the signal/electricity flows from the output source. The output impedance is the impedance of the output connectors (of the source) from where the signal/electricity flows to the input load.

This concept is similar to an audio amplifier when connecting with a speaker. As per the rule the input impedance should always be higher than the output. So, an amp with 4 ohms can be connected with a speaker of 4 ohm or higher.

Similarly, any antenna is an output to the receiver. So, the impedance of the antenna should be less than the receiver's input connectors. May be because of this reason, a matching transformer is required.

But my question is, how to find the impedance of the receiver's input?

If I connect a multimeter to the two output connectors of an antenna, I will get the output impedance, but if I do the same with the input connectors of a receiver, then am I going to get the output impedance?
In all cases, maximum signal transfer happens when impedances are matched. I argued this with my high school electric shop teacher then he proceeded to prove me wrong by filling up the blackboard with calculations. Pay attention to what others have said in this thread, resistance as in AC/DC theory has little to do with impedance and AC theory.
 
Joined
Jun 2, 2024
Messages
59
Only in the special case of a purely resistive circuit, which is generally not the case when dealing with antennas. While the unit of resistance and impedance are both the Ohm, they are not the same.



No such rule in the radio frequency world, in fact to maximize power transfer the input and output impedances should be equal.



Nope, a multimeter is not the right tool for radio frequency testing. An impedance meter or antenna analyzer would be a better choice.

I would suggest picking up a copy of the ARRL Handbook at the library & reading the chapters on electrical & AC theory, trying to use DC principles with antennas is not going to work.
What is radiation resistance, do you think?

If impedance is not resistance then why is it measured in ohms?
 
Joined
Jun 2, 2024
Messages
59
I'm really not trying to critical here, but you've shown some really elementary misunderstanding but absolute certainty that you're right? Not your fault you've not given the physics (and maths) a go - but RF is not something obvious, or even sensible.

Hopefully you will be Googling, but also consider what is called VSWR. If you have a 10W transmitter, with a length of 50 Ohm cable, and an antenna with 50 Ohm impedance, then if the antenna is tuned (as in it is resonant) at the frequency of your transmitter, then (if we ignore the bit of power that is warming up the cable) all that 10W is radiated into what is often called 'free space'. If you have a VSWR meter, or a modern electronic devices such as a VSWR capable analyser, it will show 1:1 as a reading. A perfect match. If you replace the antenna with a dummy load that doesn't radiate at all, but converts your 10W into heat - that too is a 1:1, even though nothing escapes from it. Oddly - the dummy load is actually a simple resistor - 50 Ohms. The other thing you'll discover is that if you analyse an antenna, simply putting your hand near it changes the results. Not mathematics and calculator stuff - simply waving your hand can change the readings. Even more oddly, if your transmitter is presented with a poorer load than it is happy with, it can be damaged - your 10W going one way can be reflected back up the coax cable. Your transmitter will probably notice it and throttle back, but some don't, and then overheat and die!

You can use 75 Ohm impedance cable on a receiver and it works pretty well, and is often cheaper, but you can't do it with a transmitter. The matching matters.

If you want to be even more confused, Google Alford Slot antennas. Usually found in microwave systems, this is a piece of vertical tube. A slot is milled on one side, and the coax applied half way up the slot - one connection to one side, one to the other - and it radiates HORIZONTALLY polarised, all round! A metal tube with 0 resistance. If you can understand how these work you are better than me. The maths is very complicated, but they work great!

That's enough to Google for now. See if you can get a grip on this stuff and hopefully it will begin to make sense. A resistance meter is pretty pointless with RF.
I believe that impedance is resistance and that's why it's measured in ohms. Resistance of a cable is in ohms and it also means the resistance. But I don't understand how a matching transformer would mitigate the impedance mismatch of a 300 ohm flat cable with a 75 ohm coax? That will depend on the length of the cable. If the wire is short, it will have low resistance but when it's long, it will have higher ohms.
 
Joined
Jun 2, 2024
Messages
59
In all cases, maximum signal transfer happens when impedances are matched. I argued this with my high school electric shop teacher then he proceeded to prove me wrong by filling up the blackboard with calculations. Pay attention to what others have said in this thread, resistance as in AC/DC theory has little to do with impedance and AC theory.
Whatever may be the impedance, but don't you think that a resistance can also cause signal loss?
 

paulears

Member
Joined
Oct 14, 2015
Messages
846
Location
Lowestoft - UK
You have to stop considering impedance as the same as resistance. They use the same term, but in ac circuits you have to consider capacitance and inductance too. Cable does have loss of course. However if you measure loss in a cable to RF the loss increases as frequency rises. A piece of thin coax 30m long might have nothing come out the end at 1GHz but be fine at 10MHz. Your resistance idea just doesn’t work with this stuff.
 

dlwtrunked

Member
Premium Subscriber
Joined
Dec 19, 2002
Messages
2,277
I believe that impedance is resistance and that's why it's measured in ohms. Resistance of a cable is in ohms and it also means the resistance. But I don't understand how a matching transformer would mitigate the impedance mismatch of a 300 ohm flat cable with a 75 ohm coax? That will depend on the length of the cable. If the wire is short, it will have low resistance but when it's long, it will have higher ohms.

Impedance (Z) is measured in ohms like resistance (R) but in a circuit like this antenna, impedance Z=Square_root(R^2+X^2) where X is something called the reactance which involves inductance, capacitance, and depends on the frequency. Note that from that formula, the impedance Z will always be higher than the resistance R and that is why what looks like a short is not a short. And since X depends on frequency, it is also why the antenna only has a certain range of frequencies that it is good for.
 
Top