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Size of an antenna vs. wattage and band

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desmopedro

New Member
Can you do damage to a transmitter if you have an antenna that is "too small" or the "wrong band" or length?.

I hope that is not a stupid question, but I had a discussion with a friend that I was under the assumption that if you had an antenna that was too small for a certain amount of wattage that you could damage a transmitter.

I used to goof around with CB radios as a kid and I thought I remembered you NEEDING the correct size antenna or else you may damage the transmitter.

How this came about was he was working with a VHF radio base that was used used to talk to people around an apartment complex. It was taken out of service a while back (They used Cell phones..Nex-Tel) and he was testing it by "dry keying" (That's what I remember it being called) it with out an antenna on the base station and a small hand held close by to see if he could hear his transmission. (They may hook up the system again to save money.)

He stated it didn't matter and no damage would be done.

So...I thought I would ask.

The questions I have are:

#1- Can you damage a transmitter with the wrong antenna?
#2- Can you damage a transmitter with "too small" or "too big" of an antenna? (I am SURE you can with HIGH wattage like 100 or more...but fewer than 50?) Or too large of an antenna for small amount of power?
#3- Can you damage a transmitter with the wrong "wave length" antenna or will it just hinder performance.
#4- Does band make a difference? Like you CAN damage a FM transmitter with out an antenna but you will NOT on a UHF radio.
#5- Could you use a FM/AM car antenna as a "transmission" antenna for AM or even FM transmission?

I hope these are somewhat easy questions.

Just settling a friendly dispute.

Thanks!

Happy Holidays!
 

MikeMl

Well-Known Member
Most Helpful Member
Generally speaking, it is bad practice to key a transmitter into a disconnected or any antenna which presents an impedance other than ~20Ω to 100Ω resistive at the transmitting frequency. Will you instantly destroy the transmitter? answer: low probability, but maybe.

Most modern transmitters (CB, Ham, Commercial Two Way, Aircraft) include a "VSWR Protection Circuit", which dynamically measures power reflected back to the transmitter by an improper load, and if a high VSWR is detected, it reduces the output power level to a point that prevents damage to the final stage of the transmitter.

Is it foolproof? no. Transmitting into a disconnected antenna can and has caused the replacement of many $100 transistors.

It would take a several hour lecture on antennas and feedline theory to describe all the ways an antenna can be bad at one frequency while usable on another. Suffice it to say that using an antenna designed for the band/frequency that the transmitter operates on is desirable...
 
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ke5frf

New Member
The questions I have are:

#1- Can you damage a transmitter with the wrong antenna?
#2- Can you damage a transmitter with "too small" or "too big" of an antenna? (I am SURE you can with HIGH wattage like 100 or more...but fewer than 50?) Or too large of an antenna for small amount of power?
#3- Can you damage a transmitter with the wrong "wave length" antenna or will it just hinder performance.
#4- Does band make a difference? Like you CAN damage a FM transmitter with out an antenna but you will NOT on a UHF radio.
#5- Could you use a FM/AM car antenna as a "transmission" antenna for AM or even FM transmission?

I hope these are somewhat easy questions.

Just settling a friendly dispute.

Thanks!

Happy Holidays!
#1- Can you damage a transmitter with the wrong antenna?

A. ABSOLUTELY. See what Mike said above.

#2-Can you damage a transmitter with "too small" or "too big" of an antenna? (I am SURE you can with HIGH wattage like 100 or more...but fewer than 50?) Or too large of an antenna for small amount of power?

Too small or too big covers the first question. But don't confuse power with frequency. And lets clear up some things, "large" and "small" are too vague. If by size, you mean the LENGTH of the elements, this is a frequency dependent dimension, not a power dependent. Element length is a function of frequency. If by small or large you mean element diameter, then yes, a kilowatt of power transmitted into an antenna made of 22 gauge wire might result in a meltdown of both the antenna and the transmitter amplifier.

#3-Can you damage a transmitter with the wrong "wave length" antenna or will it just hinder performance.

This would be covered by being the "wrong antenna". The right antenna would solely be the antenna of the appropriate length for the frequency (i.e. wavelength/reciprocally) of operation, and made thick enough for the operating power...(which shouldn't be more than a few watts on CB BTW so it would be hard to make an antenna too "thin")

#4- Does band make a difference? Like you CAN damage a FM transmitter with out an antenna but you will NOT on a UHF radio.
#5- Could you use a FM/AM car antenna as a "transmission" antenna for AM or even FM transmission?

Put these two together for a reason. FM is not a band first of all, neither is AM. They are modulation types. For instance, you can very well, and most likely DO, have a UHF FM radio. FM is the customary voice mode on UHF. However, it doesn't HAVE to be. Antennas have nothing to do with modulation types. Modulation type is not frequency dependent (well not scientifically or electronically speaking but perhaps legally they are)...Being not frequency dependent, there is no such thing as an FM antenna.

However, certain modulation types customarily adopt certain POLARIZATIONS for antennas. For instance, FM is generally vertically polarized while AM and sideband can be either/or. This is only a convention, though, and not a requisite of operation.


BTW, I would thoroughly encourage you to read up on radio theory on legitimate AMATEUR RADIO websites and dedicated electronics websites that don't incorporate names like "Rubber Ducky's Intar-Net 'Lectronics Learnin' site" or some such. :D I'm not saying a few Rubber Duckies and Mud-duckers out there don't know a thing or two about radio, but if you spend more than a minute or two listening to or reading their CB material, you stand a chance of getting a whole lot of misleading CRAP and flat out wrong information. You'll also pick up on a lot of lingo that is mind numbing. You don't sound like a present day CBer but I'm just forwarning you about the pittfalls.
 

audioguru

Well-Known Member
Most Helpful Member
I was given a cheap Chinese RC model airplane for Father's Day.
I tried it a few times with the transmitter antenna extended and it worked fine.
A visitor turned on the transmitter with the antenna retracted and it instantly was destroyed and began smoking. Its power was only maybe 1W.

The importer replaced the transmitter.
 

EN0

Member
I was given a cheap Chinese RC model airplane for Father's Day.
I tried it a few times with the transmitter antenna extended and it worked fine.
A visitor turned on the transmitter with the antenna retracted and it instantly was destroyed and began smoking. Its power was only maybe 1W.

The importer replaced the transmitter.

1W?! That seems hard to believe at such low power even if you do have a high SWR.
 

PatM

Member
It is also important to remember which mode you are operating.
FM is 100% duty cycle and can cause damage to the finals easier than when operating SSB or one of the digital modes.
 

desmopedro

New Member
Very cool! Thanks a TON!

I know there is a LOT to learn on this stuff and your answers make me realize how clueless I actually am to this stuff! :) I have read a number of posts on things I would LOVE to try! (The one I really liked was the one about a guy trying to set up an FM transmitter to broadcast Christmas Music for passers-by! I have a NEIGHBORHOOD full of Christmas lights...and as the prior owner of a DJ company, I would LOVE to provide passer by's music they can tune into!)

For as much as I LOVED to "play" with CB radios as a kid....I would LOVE to get into some kind of amateur radio hobby in the future. My home is in a PRIME location as I live in a HIGH elevation area compared to my surroundings!

If anyone knows of a good book to read as far as a "beginners guide" to understanding how this stuff works...I would be happy to take your advice and hit up Amazon for some winter vacation reading.

Again thanks a ton all! You all have been most helpful! I just hope I can contribute to the site now....
HAPPY HOLIDAYS!
 

mbarazeen

Member
i have switched power of some 6W, 12W FM transmitters without antenna but for few minutes. i got the final stage amp over heated during this, when its connected to antenna it worked cool.
i couldnt beleive a 1W power damaged the atransmitter?

length of antenna elements(size) plays the major role of maching it with open air and when it maches it radiates 100% of the pushed (forword) power from transmitter. if not it reflect a part of it back, or some cases all of the power.

current rating of the element( for radiation) and components used for maching inside the antenna if any decides its capacity - max power.
 
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ke5frf

New Member
Actually, and this is another thing you will find out if you take further interest in amateur radio as a hobby, elevation isn't of particular consequence unless you are on a band above around 50 MHz (VHF and above). The Amateur HF bands from 1.8 MHz to 29.7 MHz (this isn't continuous but rather spans 10 seperate small bands of a few MHz each) perform quite well at almost any elevation...in fact sometimes high elevations and hilly terrain are detriment and best operation is achieved near sea-level.

Of course, those bands are limited with progressively larger privelages for licensed operators. In the US, Technician class was recently granted very limited HF privelages, and the old Novice licensees retain the same limited privelages. General class gets a significant chunk more, Old "Advanced" class licensees get still a bit more, and Extra class (of which I am licensed) get the entire amateur allocation on all bands.

VHF and above are granted to all licensees with equal access EXCEPT novice class, which is no longer granted as a license. (Novice and Advanced are grandfathered).

If you are interested in learning and want a beginner introduction, "Now You're Talking" published by the ARRL is a pretty good general introduction into amateur radio Technician Class licensing. You have not had to learn Morse Code for about two years for any license since the rule changed. I licensed under the 5 WPM rule but can operate at 30 WPM. I HIGHLY encourage anyone interested in amateur radio to learn Code. Yes, it is archaic but it is an ART that is indescribeable until you start mastering it. I admit, I am kind of snobby about ham radio, I do not believe a ham is truly a ham unless he has AT LEAST made the effort to learn the 5 WPM standard, even if it is to just listen or recognize automatic beacons transmitting their IDs in Morse.

But I digress, it is interest in radio that really counts and I encourage you to pursue it a little further with a fresh slate and willingness to forget the CB stuff.
 

ke5frf

New Member
1W?! That seems hard to believe at such low power even if you do have a high SWR.

Nerdy,

Consider that a 100 watt light bulb produces 90% waste heat. Consider how hot this is, 90 watts of heat.

Now, consider that an RF amplifer is much more efficient when transmitting into a resonant antenna, but it still produces a certain amount of waste heat. I want to say a typical RF amplifier is efficient to the positive side, lets say 75% efficient for arguments sake, with 25% as loss.

That would be 25 watts of heat for a 100 watt amplifier. Now, when the antenna isn't resonant, the reflected power along with resistive losses might create a condition where the amp is as little as 10% efficient or worse. I'm not referencing theoretical numbers, I'm not an amp designer, but the ballpark is reasonable enough. So, without a foldback circuit this amp will cook very quickly.

high power RF amps are bulky, sturdy, heatsinked devices. A TO-66 or similar package would be expected. Now, consider a low power RF amplifier in a TO-92 package. Consider it only produces, perhaps, 3 watts. Now, consider this small amplifier transmitting into a non-resonant antenna. Consider that it will likely be a less efficient amplifier design in the first place, being a cheap toy with design constraints. Consider that it may already be dissipating 2 watts or more of heat by way of being a poor design. Now, consider reflected power and resistive loss adding another watt or two of wasted heat.

Now, you might very well be dissipating 3 or 4 watts of heat waste in a tiny TO-92 package with no heatsinking or cooling fan. A typical bathroom nightlight might generate 3 or 4 watts of heat. Have you ever felt a nightlight? For such a small bulb, they get pretty warm, and they have the benefit of a larger cooling medium and exposure to air circulation most of the time. So, it isn't surprising that a tiny RF amplifier in a toy can burn up just as easily transmitting into an incorrect impedance.
 
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