Antenna length efficiency technical question

[kb5udf]

For the techies/elmers out there I had a thought. Since in theory,
radio waves hit the antenna, thereby energizing it, what is the
practical difference that size makes?

Specifically, a 1/4 wavelength vertical at VHF (say 155mhz) versus,
a 1/4 wavelength vertical at 860 mhz; coax losses aside, is there any pracitcal or theoretical difference in received signal strength.

If not, how different do two frequencies have to be for signals of equivalent power to be received at different levels by appropriately tuned antennas. Ie, would a 1/4 wave vertical at 28 mhz, have more signal strength (for a 28mhz signal) than a 1/4 wave verticaal at say, 2.8 ghz.

 

[crayon]

If not, how different do two frequencies have to be for signals of equivalent power to be received at different levels by appropriately tuned antennas. Ie, would a 1/4 wave vertical at 28 mhz, have more signal strength (for a 28mhz signal) than a 1/4 wave verticaal at say, 2.8 ghz.

No difference in actual mechanics of the antenna.

However, bear in mind that you cannot compare the antennas side-by-side. RF intensity diminishes with the square of distance, and microwaves are line-of-sight.

 

[N_Jay]

For the techies/elmers out there I had a thought. Since in theory,
radio waves hit the antenna, thereby energizing it, what is the
practical difference that size makes?

Specifically, a 1/4 wavelength vertical at VHF (say 155mhz) versus,
a 1/4 wavelength vertical at 860 mhz; coax losses aside, is there any pracitcal or theoretical difference in received signal strength.

If not, how different do two frequencies have to be for signals of equivalent power to be received at different levels by appropriately tuned antennas. Ie, would a 1/4 wave vertical at 28 mhz, have more signal strength (for a 28mhz signal) than a 1/4 wave verticaal at say, 2.8 ghz.

2nd year fields and waves! Ouch!

The comparison you are trying to make is very difficult.

There are two many things to consider.

Are you talking about the two frequency fields being the same streangth in mV/Meter, or the two fields being gererated by transmitters with the same ERP at the same distance?

 

[kb2vxa]

Hi UDF and readers,

>GULP< OK, let's try to untangle this mess. (;->)

"For the techies/elmers out there I had a thought. Since in theory,
radio waves hit the antenna, thereby energizing it, what is the
practical difference that size makes?"

That's a VERY ambiguous question with many possible answers. Here goes nothing. The length of let's say one active element to keep it simple determines the feed point impedance for one thing, assuming it is resonant. Resonance occurs when the lenth of the element matches either 1/4 or 1/2 the wavelength in free space (not considering end effects which electrically lengthen the radiator so it must be shortened accordingly) or is an odd multiple of 1/4 wave or an even multiple of a 1/2 wave which provides the same relative impedance in each case. There's more but I'm not rewriting the ARRL Antenna Handbook, good reading.

"Specifically, a 1/4 wavelength vertical at VHF (say 155mhz) versus,
a 1/4 wavelength vertical at 860 mhz; coax losses aside, is there any pracitcal or theoretical difference in received signal strength."

No. That's the simplistic answer if the field strength in uV/M is the same, otherwise things can get complicated.

"If not, how different do two frequencies have to be for signals of equivalent power to be received at different levels by appropriately tuned antennas. Ie, would a 1/4 wave vertical at 28 mhz, have more signal strength (for a 28mhz signal) than a 1/4 wave verticaal at say, 2.8 ghz."

UG! People use the term "power" so loosely an accurate yet simple answer cannot be provided. Then too you're asking a question that in order to give a specific answer I would fill this page with mathematical equasions. If you're talking as in the above answer it's already been given, if otherwise things can get messy. Another simplistic answer is this, provided the effective radiated power (ERP) of the transmitters is the same the difference lies in propagation path losses. To put it simply you get fewer miles per watt as the frequency goes up. Since a quarter wave regardless of frequency has unity gain the received signal at 28MHz would be far stronger than at 2.8GHz for the same ERP at the same distance over flat terrain. When it gets lumpy things get AWFULLY complicated.

Yours has GOT to be a vanity callsign. (;->) Don't comment, let's keep 'em guessing.

 

[crayon]

kb2vxa, your posts would be much more easier to read if you would start using the tags to help us know what it is you are quoting and what is your comments.

Be sure that you have BBCode is enabled in your control panel. The syntax is:

words letters sentences paragraphs etc

Thanks for your future attention to this minor detail!

 

[N_Jay]

For the techies/elmers out there I had a thought. Since in theory,
radio waves hit the antenna, thereby energizing it, what is the
practical difference that size makes?

Specifically, a 1/4 wavelength vertical at VHF (say 155mhz) versus,
a 1/4 wavelength vertical at 860 mhz; coax losses aside, is there any pracitcal or theoretical difference in received signal strength.

If not, how different do two frequencies have to be for signals of equivalent power to be received at different levels by appropriately tuned antennas. Ie, would a 1/4 wave vertical at 28 mhz, have more signal strength (for a 28mhz signal) than a 1/4 wave verticaal at say, 2.8 ghz.

Hmmmm ?

Callsign: KB5UDF Class: Advanced

 

[crayon]

Callsign: KB5UDF Class: Advanced

Brain dump'd the test?

 

[N_Jay]

You mean like this

kb2vxa, your posts would be much more easier to read if you would start using the tags to help us know what it is you are quoting and what is your comments.

Be sure that you have BBCode is enabled in your control panel. The syntax is:

words letters sentences paragraphs etc

Thanks for your future attention to this minor detail!

 

[crayon]

You mean like this

Yeah.

The problem is that one cannot actually show the BBCode syntax *and* the end result of using the aforementioned syntax in the same post without producing the effects of said syntax.

 

[N_Jay]

You mean like this

Yeah.

The problem is that one cannot actually show the BBCode syntax *and* the end result of using the aforementioned syntax in the same post without producing the effects of said syntax.

Just giving you a hard time.

No Prob

 

[crayon]

Just giving you a hard time.

lol. I no.

 

[kb5udf]

Question Clarification/Simplification

I'm afraid my original post did not generate the information I was looking for.

So here goes:

Given two unity gain 1/4 vertical radiators, one at 28 mhz, and one at 2.8 ghz, both being fed with 10 watts RMS of power.

Also given two receiving antennas, of identical specifications, let's, say 20 feet way, in a perfect environment w/out obstructions,

Also given perfect coax(no losses) and receivers of identical sensitivity/dynamic range, (ie theortically perfect)

Q: Would there be any difference in received signal strength between the 28mhz and 2.8 ghz signals? ie, do vertical 1/4 wave antennas become somehow less efficient as they greatly decrease in size?

 

[crayon]

Re: Question Clarification/Simplification

Q: do vertical 1/4 wave antennas become somehow less efficient as they greatly decrease in size?

Given the isotropic nature of the test bed, I would say that there is no change because the physics has not changed. A 1/4 wave antenna will resonate at the frequency it was cut to. Be it a UHF, VHF, or even a microwave frequency.

As already noted, real world application affects performance.

Good question.

 

[N_Jay]

Re: Question Clarification/Simplification

I'm afraid my original post did not generate the information I was looking for.

So here goes:

Given two unity gain 1/4 vertical radiators, one at 28 MHz, and one at 2.8 GHz, both being fed with 10 watts RMS of power.

Also given two receiving antennas, of identical specifications, let's, say 20 feet way, in a perfect environment w/out obstructions,

Also given perfect coax(no losses) and receivers of identical sensitivity/dynamic range, (ie theoretically perfect)

Q: Would there be any difference in received signal strength between the 28mhz and 2.8 GHz signals? ie, do vertical 1/4 wave antennas become somehow less efficient as they greatly decrease in size?

10 Watts power in to a theoretical 0 DB (Unity gain) antenna system will give you 10 W ERP (+40 dBm) no matter what the band is.

The receive signal strength to a similar antenna set up would be the path loss subtracted from the the +40 dBm.

The path loss would be slightly different depending on the band, but at short distances the differences would be minor. Again, the difference would be due to the propagation, not the antenna efficiency.

One little note. At very short distances the two antennas could be in each others near field. Since near field effects are wavelength related this is more likely to happen at lower frequencies.

(Just another one of the real world things that screws up tests based on theoretical principles.)

 

[kb2vxa]

Hi again UDF and all,

To quote myself, this is the answer to your question, I don't know why you asked twice.

"Since a quarter wave regardless of frequency has unity gain the received signal at 28MHz would be far stronger than at 2.8GHz for the same ERP at the same distance over flat terrain."

There it is succinctly, if you're still confused or I'm unclear e-mail me so we can hash it out to your satisfaction.

Now Crayon, I quote the way I was taught in school by using quotation marks. Sorry if they confuse you. Call me an old fuddy-duddy, I don't rely on a computer to do my writing composition for me. I'm politically incorrect and proud of it too. Maybe if you put down the crayons and

Nah, I can't say it, stop fingers STOP! Now get back in your cage and leave the poor kid alone. (;->)

 

[N_Jay]

. . . . .
To quote myself, this is the answer to your question, I don't know why you asked twice.

"Since a quarter wave regardless of frequency has unity gain the received signal at 28MHz would be far stronger than at 2.8GHz for the same ERP at the same distance over flat terrain."

There it is succinctly, if you're still confused or I'm unclear e-mail me so we can hash it out to your satisfaction. . . . .

Very clear, not correct, but very clear.

(Again, it will be somewhat stronger but due to the path loss, not the antenna.)

Unity gain is unity gain, at 28 MHz or at 28GHz.

Why do you think it will be FAR stronger at lower frequencies?

Don't just say it is, tell me WHY! :wink:

 

[crayon]

Since a quarter wave regardless of frequency has unity gain the received signal at 28MHz would be far stronger than at 2.8GHz for the same ERP at the same distance over flat terrain.

In the context of the question, this is an incorrect conclusion. Your trying to explain the effect that DISTANCE has on how many micro-volts will be induced into an antenna.

The question piviots around the concept of comparing the EFFICIENCY of a 1/4 wave antenna @ 28MHz vs. the EFFICIENCY of a 1/4 wave antenna @ 2.8GHz. This is a very astute question from a thinking person because it is beyond debate that a 1/4 wave antenna @ 28MHz is l-o-n-g-e-r than one @ 2.8GHz.

To rehash it again: A 1/4 wave antenna @ ANY frequency is a 1/4 wave, is a 1/4 wave, is a 1/4 wave, is a 1/4 wave, is a 1/4 wave, is a 1/4 wave, is a 1/4 wave, is a 1/4 wave.

Frequency has no bearing on the performance (read the reasonating performance here) of a 1/4 antenna as stated quite well by N_Jay:

10 Watts power in to a theoretical 0 DB (Unity gain) antenna system will give you 10 W ERP (+40 dBm) no matter what the band is.

I don't rely on a computer to do my writing composition for me.

Actually, you do. Without a computer you would not be able to post your comments to this forum.

My request that you use the quote syntax when perparing your comments was a request of courtesy. It is much like holding the door for someone, we all know that we are not obligated to hold the door for anyone, but we do it anyway. However, when it is done for us, or when we do it for others ... it is always appreciated.

Any given thread tends to have several members discussing a topic and the posts can span many pages. If you decided to respond to different comments in one post, how are we to know who you are talking too?

I've pointed out to you what most people in the community generally accept as the protocol, <shrug> how you will choose to post in the future after this makes no difference to me. I aint the one who looks like a newbie.

I'm politically incorrect and proud of it too.

There are far more important things in life to be proud of than a personality trait one may possess.


edit -- ack! N_Jay slipped a post in on me whilst I was composing ..

 

[N_Jay]

edit -- ack! N_Jay slipped a post in on me whilst I was composing ..

That's because you were trying to say too much!

 

[crayon]

That's because you were trying to say too much!

lol.

 

[kb2vxa]

Hi NJay and all,

"Unity gain is unity gain, at 28 MHz or at 28GHz. Why do you think it will be FAR stronger at lower frequencies? Don't just say it is, tell me WHY!"

Well, I did in my original post but here it is again as a stand alone for a bit more clarity. The key phrase (without all the math) is "propagation path loss". The higher the frequency the greater the loss so all things being equal a 28MHz signal would be considerably stronger than a 2.8GHz one. You get roughly a mile per watt on 2M and VHF Hi Band just so you have a point of reference.

Just to confuse you I'll regurgitate a few details. Each signal source radiates with the same power ERP or effective radiated power and each receiving antenna has a particular gain each the same, say unity as not to overcomplicate things. The field strength starts out the same but by the time it gets to the receiving antenna it diminishes by the square of the distance plus propagation path loss. This being attenuation increases with frequency it means the 28MHz signal will register more microvolts per meter (uV/M) than the 2.8Ghz one. Since the receiving antennas each have unity gain they will (not counting differences in transmission lines) deliver a signal to the receiver accordingly. (The guy on 28 gets a 5/9 report while the one on 2.8 gets a 3/2.) Oh, ya gotta learn HAMSPEAK! (;->)

Bottom line here is the same thing happens in air as does in coax, only space is immune. Yup, out there all RF signals diminish by the square of the distance only. BTW, of course this also applies to light the same as anything in the specrum so that's one of the ways astronomers can tell how far a way a star is. Hmmm, I wish I had my friend Armand here, he could explain much better than I and tell you the Doppler shift too.

I guess that's why the SHF guys use loop Yagis while the 10M ones get away with dipoles.

Oh Crayon, go take a hike, will ya?