Antenna Impedance

[n211cr]

I was looking into seeing what I can pick up during the propagation of this capsule returning Sunday morning. I have an FM receiver that requires a 300 ohm impedance antenna connection. I plan on using some 75 ohm coax to a Yagi which I am going to put together. Do I need to get a transformer to both sides, or I am assume only at the FM receiver. Should I be using 50 ohm when connected to a homebuilt yagi. I am just confused on if the coax ran and what the receiver requires is supposed to match or if the antenna added a third item that has to be compatible. I may be wrong in saying it is only the feed line and receiver, unless using an unbalenced antenna?

 

[N_Jay]

if the yagi is 50 -75 ohm, then you do not need a xformer at that end, if the radio is 300 ohm then you DO need one at that end.

 

[n211cr]

Ok Right I have that, I am building a yagi, how do I tell which its going to be, what are the design differences?

 

[KC4ZEX]

Not sure what design you are using for the yagi. Most yagi's built like tv antenna's are designed for the 300 ohm feed. But most yagi's built for amateur radio, CB, or other business radio systems have a matching system such as gamma match or balun at the feed point, which usually requires 50 ohm coax.

 

[N_Jay]

Not sure what design you are using for the yagi. Most yagi's built like tv antenna's are designed for the 300 ohm feed. But most yagi's built for amateur radio, CB, or other business radio systems have a matching system such as gamma match or balun at the feed point, which usually requires 50 ohm coax.

Most TV antennas are Log Periodic, not Yagis.

It depends on the driven element. Yagis driven by simple dipole elements are usually 50 ohms.

 

[johnnye]

Ok I have a couple similar questions so I'll post it here since it seems on topic. I'm also homebrewing a yagi antenna, for 800mhz (specifically 867). I just got my 9913 last weekend, so I want my yagi to run at 50Ohm. I guess 200 is possible, I picked up a 4:1 balun while I got the 9913. It's nice to live 30 min from HRO in Woodbridge.
So the questions are, I'm using QY4 to design my yagi. It shows your input impedance for the antenna you designed, such as 16.8 -j 0.0 Ohms. If I fiddle with the fed dipole length I can get the ohms to be 50, but for some reason I think I'm doing something wrong.

I'm going to try to attach screenshots of QY4 to show what I mean. The first one is output it gave me in auto mode, and the second is when I changed the fed dipole length so that the Ohms matched 50. Is either of these methods right? And if the first way is correct, how do I connect it to 50 Ohm 9913?

 

[n211cr]

So as shown here is the impedance a factor of there being a balun or some sort of matching device, or is it in design. What would be the difference of the driven element in a 300 ohm, a 75 ohm, and a 50ohm, only size? Also I am going to be using this for RX only.

 

[Pro-95]

Although in a perfect world you would want to have i(resistance)=50 and j(reactance)=0, this is typically not practical. To get the antenna without a matching network to this specification would require compromises. Every commercially available Yagi has some sort of matching network. The first i=xx and j=0 with the best design specifications (gain,F/B,sidelobe,bandwith,beamwidth, etc) is usually the design built then matched using a gamma match. Resistance is the easiest to adjust for but (so I've been told) adjust for the least amount of reactance. You can also work on reducing the reactance with a decoupling loop.

A 50 ohm going to a 4:1 balun is 200 ohms. A lot closer to 300, 450 or 600 than 50 is.

Remember that a computer model is in a "perfect" world and where you will be using your design is not. There are a ton of variables from ground conductivity, feed line quality, quality of construction, material deviations to precision of measurements that will impact the actual characteristics of the antenna. Just build it close then match using a matching network. Almost every manufactured antenna has a matching network and will require the end user to make adjustments for the environment that the antenna will be used.

 

[Al42]

It's nice to live 30 min from HRO in Woodbridge.

You evidently don't have to park (oh, excuse me - they call it "driving", don't they?) on US1 during the rush hour.

It shows your input impedance for the antenna you designed, such as 16.8 -j 0.0 Ohms. If I fiddle with the fed dipole length I can get the ohms to be 50, but for some reason I think I'm doing something wrong.

That's not the resistance ("the ohms") you're looking at, it's the reactance. Ideally you want 50j0. Which you're not going to get with a Yagi.

You could look at using a quad driven element. That'll should be adjustable to about 208j0, which you can match with a length of ladder line. (Any quad design site will show the matching network usually used.) And it'll increase the gain.

 

[kb2vxa]

Hi guys,

All things considered I don't know why you got all hyped up over an event lasting a few seconds and went after it without a plan, without rhyme or reason. Be that as it may this has turned into an antenna discussion without anyone asking the question of what sort of a receiver having a 300 ohm impedance would be used and for what. In the end the poor guy was left twisting in the wind.

"Ok Right I have that, I am building a yagi, how do I tell which its going to be, what are the design differences?"
Chad, you have a lot to learn and it's far more than can be acomplished here. Being you're a ham you must have heard of the ARRL. I don't want to sound sarcastic but by the looks of it you all are in way over your heads so start cracking the books before you go crazy wondering why that contraption doesn't work as you expected. Here are a few examples.

"Most TV antennas are Log Periodic, not Yagis."
I'll disagree, they vary widely and actually only a few are logs. Some ARE Yagis, they're high gain deep fringe single channel VHF or UHF antennas. BTW, I once had boxes full of them for channel 68 and wish I had them now, just look up the frequency chart. Don't argue, I didn't spend all those years in the business for nothing. Actually I did it for the money.

"It depends on the driven element."
Sometimes.

"Yagis driven by simple dipole elements are usually 50 ohms."
A dipole is 75 ohms which is why those TV Yagis came with short RG-6U jumpers and F connectors. A 50 ohm Yagi has a gamma matched electrically continuous grounded driven element while one for 75 ohms has one split, center fed and insulated from the boom.

"I'm also homebrewing a yagi antenna, for 800mhz...I want my yagi to run at 50Ohm. I guess 200 is possible, I picked up a 4:1 balun while I got the 9913. It's nice to live 30 min from HRO in Woodbridge."

You started off on the right foot, then you tripped over it. If you want 50 ohms, MiniNEC for 50 ohms. OK, 200 is possible but totally impractical, then you bought the wrong balun. I know of no baluns for 800MHz besides TV baluns and they're 4:1 but then HRO sells HF baluns and they get flaky above 30MHz.

"If I fiddle with the fed dipole length I can get the ohms to be 50, but for some reason I think I'm doing something wrong."
You're doing a LOT wrong, feedpoint impedance is affected by a lot more than the length of the driven element and that determines the resonant frquency more than anything else. It's the feedpoint >along< it that determines the impedance more than it's overall length. Dr. Smith says "Oh the math, the MATH!" No, I'm not Lost In Space, I'm talking about the Smith chart. There are other things to consider like the ratio of it's length to the reflector and first director, element spacing and more. Then there is beamwidth, forward gain, front to back and front to side ratios, the whole polar pattern is affected by these factors.

Al, stick to antennas, you can't read a map with an interpreter. (;->) You have US-1 and Woodbridge right but neither Johnny nor HRO are in New Jersey. Now rather than going through all that 50-JO stuff why didn't you suggest a loop Yagi? Heck, a bunch of cut up beer cans on a broomstick may look like some sort of sci-fi Bazooka but it works. And no, that wasn't a typo. (;->)

Now is when I really wish I still had that box of antennas. I could offer them free of charge, just come and pick one up. Here's your antenna >>>WHACK<<< an' thet's fer gittin' so gol durn tek nee kull on me! Consider the herringbone welt you get from the antenna over your head the KB2VXA brand, the mark of quality, so wear it with pride. (;->)