Newbie Question -- I don't understand "Impedance Matching"

[hackinstack]

I've looked on the internet and read numerous different sites and I'm still struggling with the concept of "Impedance Matching" in regards to antenna design. Can someone explain it in layman's terms. It seems like all the information I find quickly degenerates into a physics lesson or a small EE course.

I just want to know simply...

- What effects an antenna impedance value? (size, shape, all of the above, etc...)
- How do I determine the impedance of an antenna? (calculators? do I need a commercial antenna tuner?)
- What can I do to correct the impedance value of an antenna? (passive components? specific types of circuits?)

Again, apologies for the basic question, but these answers seem to be eluding me...

 

[N_Jay]

Technical answer:
1) All of the above
2) Design calculation for an estimate, and multiple ways with test equipment to measure.
3) Adjust antenna, add components, make transformers.

Good enough answer for most receiver applications:
1) antenna design
2) antenna design
3) don't worry about it.

 

[W6KRU]

I've looked on the internet and read numerous different sites and I'm still struggling with the concept of "Impedance Matching" in regards to antenna design. Can someone explain it in layman's terms. It seems like all the information I find quickly degenerates into a physics lesson or a small EE course.

I just want to know simply...

- What effects an antenna impedance value? (size, shape, all of the above, etc...)
- How do I determine the impedance of an antenna? (calculators? do I need a commercial antenna tuner?)
- What can I do to correct the impedance value of an antenna? (passive components? specific types of circuits?)

Again, apologies for the basic question, but these answers seem to be eluding me...

I'm no engineer myself but I'll take a quick cut at this and then the more knowledgeable types can fill in gaps.

Size and shape affect the impedance and at lower freqs. the height above ground can as well.

You do need elaborate measuring tools to determine impedance of an antenna and this can be difficult stuff to understand. It is for me anyway.

Correcting the impedance is a very specific thing and changes with the situation.

 

[hackinstack]

"3) don't worry about it."

Does this mean I dont have to worry about frying a receiver if I plug in an antenna that is not correctly matched? This is my main concern as I would like to make some better antenna's but want to make sure I don't cook my equipment by accident.

 

[W6KRU]

I thought Jay would pop back in to answer your question but I'll go ahead. No, you will not hurt your receiver buy using a mis-mateched antenna or feedline. You just won't have optimum reception.

 

[AlmostHandy]

What kind of test equipment will measure impedance?

I'm just curious.

 

[W6KRU]

What kind of test equipment will measure impedance?

I'm just curious.

For the average hobbyist an MFJ-269 is probably the best thing out there for the price. I'm no fan of MFJ stuff but I have a 259B. The 269 model adds uhf but it doesn't do anything over 470 Mhz.

 

[hackinstack]

"No, you will not hurt your receiver buy using a mis-mateched antenna or feedline. You just won't have optimum reception."

This puts me much more at-ease about tinkering with making my own antenna's. Thanks!

 

[kb2vxa]

Considering the theory you're asking about is very complex there is no simple answer. There are Amateur Radio publications available that put it clearly but simply, beware, there is math involved. That's about the best suggestion I can make short of sending you off to a university for an electrical engineering degree (sarcasm) but you may be interested in some old used books on it you can find in bookstores if you're so inclined.

Cutting all that to the bone, for receiving only and general listening applications as long as impedance isn't ridiculously out of line it really doesn't matter. The antenna input on the average receiver will accept a broad range without complaint. It might explode in flames however if you hook up a power line for an antenna. That being an unknown impedance who knows what will happen? (sarcasm again) What matters is resonance but that's another question and another can of worms.

 

[KC4RAF]

Short term for Impedance

is the resistance of the antenna to the given radio for a specific radio frequency. For receiving ONLY, you don't have to worry too much about the impedance. If you are transmitting, then you could blow the finals if the impedance is too great. Usually you have 50 ohms or maybe 75ish ohms for many of the scanners. As others have noted, it's not too much to worry about unless you get way out of match.

 

[buddrousa]

As stated it will not hurt your scanner but it would kill a transmitter. A mismatch is like pushing water through a rubber lined hose and a matched line is like pushing water through a steel pipe. Friction verses no friction.

 

[hackinstack]

Thanks again for the input guys. I picked up a copy of the latest ARRL handbook and have been attempting to digest the antenna section. (amongst other things as it is a really good reference) Right now I'm just playing around with an Icom-R5. I've been toying with the idea of a little better receiver, but for now -- no transmitting in my fore-seeable future.

 

[kb2vxa]

Here's a good example of misinformation commonly found on this forum:
"Short term for Impedance is the resistance of the antenna to the given radio for a specific radio frequency."

Negatory good buddy, radiation resistance is a horse of another color entirely and another complex issue. Please don't confuse DC resistance with AC impedance and don't think radiation resistance has anything to do with either.

See now why I said that short of a BSEE what you need are some Amateur Radio publications and some "engineering lite" study manuals? The subject is WAY beyond the scope of a discussion forum yet they continue long after the thread should have petered out. Oh well.

"...no transmitting in my fore-seeable future."
That's too bad, you've just taken the first step. BTW, the word is "foreseeable". (;->)