# High frequency antenna design

David lopez
if an antenna 8.2 feet long was coiled up into a coil could it still receive radio signals at 30 megahertz?

Mentor
if an antenna 8.2 feet long was coiled up into a coil could it still receive radio signals at 30 megahertz?
Sure. I can receive those signals with my tooth fillings.

Can you post the antenna equations that apply to your question please?

dlgoff, Tom.G and gneill
8.2 feet is 2.5 metre which is λ/4 at 30 MHz.
I assume you would mount and feed it as a whip antenna, perpendicular to a ground plane.

If you coil the same length of bare wire into a helix, and mount it on the same ground plane, it will still be resonant at 30 MHz, but the impedance and the radiation pattern will change.

David lopez

Gold Member
The small antenna has very small radiation resistance - in other words, it does not couiple verty well to the incoming wave. If we can extract all the power from this small resistance, fine, but the resistance of the copper wire is large by comparison and will absorb most of the energy.

David lopez
Sure. I can receive those signals with my tooth fillings.

Can you post the antenna equations that apply to your question please?
i used
The small antenna has very small radiation resistance - in other words, it does not couiple verty well to the incoming wave. If we can extract all the power from this small resistance, fine, but the resistance of the copper wire is large by comparison and will absorb most of the energy.
so it won't receive a signal, because the resistance of the copper wire will absorb most of the energy?

Mentor
i used
Hmmm.

Anyway, 30MHz is not normally considered "high frequency". High frequency for RF and antennas is more like 1GHz, but it's all relative I guess.

See if this previous thread is of any help on your question:

There are reasons that ferrite Rx antennas are used at low frequencies like 1MHz... You can do a web search to see what frequency ranges you can get ferrite antennas for, to see if they might work in your application...

David lopez
The small antenna has very small radiation resistance - in other words, it does not couiple verty well to the incoming wave. If we can extract all the power from this small resistance, fine, but the resistance of the copper wire is large by comparison and will absorb most of the energy.
so it won't receive a radio signal, because the resistance of the copper absorbs most of the energy?

Gold Member
i used

so it won't receive a signal, because the resistance of the copper wire will absorb most of the energy?
Yes. Let me talk about a transmitting antenna because it is easier to explain. When we reduce the size of the antenna, the radiation resistance reduces very drastically. Provided we can couple all our transmitter power into that small resistance, we still have good radiation. But the problem is that the loss resistance of the antenna now becomes significant and absorbs power.

Mentor
if an antenna 8.2 feet long was coiled up into a coil could it still receive radio signals at 30 megahertz?
I think the OP is asking about an Rx antenna, but I could be wrong...

David lopez
i don't know what an Rx antenna is?

Mentor
The opposite of a Tx antenna (unless they are the same thing).

David lopez
oh, so a Rx antenna is a receiver.

berkeman
Mentor
And after reading that other thread that I linked, can you say more about why Rx and Tx can use different antenna structures?

What is your application? You want to receive 30MHz transmissions with an electrically small antenna? That's pretty easy, if that's what you want to do.

alan123hk
if an antenna 8.2 feet long was coiled up into a coil could it still receive radio signals at 30 megahertz?

In the past, as an electronic product designer, I encountered the same difficulties several times. For a variety of reasons, such as better product appearance, managements always want to shrink antenna size endlessly in order to downsize their products.

However, managements usually have no engineering background and know little about the antenna, they think that a long linear wire antenna can be coiled up into an equivalent coil with arbitrary short length as wish, and naively believe that this is the golden rule to solve every problem.

Unfortunately, things in the world are usually not so simple and easy, otherwise, there wouldn't be so many products with long rod antennas on the market.

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David lopez
And after reading that other thread that I linked, can you say more about why Rx and Tx can use different antenna structures?

What is your application? You want to receive 30MHz transmissions with an electrically small antenna? That's pretty easy, if that's what you want to do.
so how do you receive 30 mhz signals with an electrically small antenna?

Mentor

David lopez
somebody posted on the other thread
"The antenna length tells you the frequency at which the antenna will resonate, and thus be most efficient. Antennas used for frequencies other than their resonant frequency will often use a matching network of some kind to get better efficiency"
see i read the other thread. does this mean a 3 foot long antenna could pick up 30 megahertz signals? any way to make this work?

Gold Member

But you didn't understand ...

does this mean a 3 foot long antenna could pick up 30 megahertz signals?

It will pick up signal, but it will be very inefficient (very poor) compared to an antenna that is a resonant length

you still haven't answered @berkeman 's question ... what is your application ?

David lopez
a device that will measure the temperature in my home, when i am not home, has to be when i am not home, and sent a message over long distance, which is why i use 30 megahertz.

David lopez
by the way if i use a matching network, will i get enough efficiency to obtain certain information, temperature readings, clear video and clear audio?

Staff Emeritus
a device that will measure the temperature in my home, when i am not home, has to be when i am not home, and sent a message over long distance, which is why i use 30 megahertz.

That's kind of old fashioned. Your device should use WiFi to connect to your local Internet and send you emails, or a cell phone connection to send you text messages. Emails and texts work at any range.

There are several kinds you can buy.
https://www.amazon.com/s?k=remote+temperature+monitor&ref=nb_sb_noss_1&tag=pfamazon01-20

davenn and berkeman
David lopez
by the way if i use a matching network, will i get enough efficiency to obtain certain information, temperature readings, clear video and clear audio?

David lopez
could you still answer this question?

Mentor
a device that will measure the temperature in my home, when i am not home, has to be when i am not home, and sent a message over long distance, which is why i use 30 megahertz.
could you still answer this question?
There are no unlicensed bands around 30MHz near your location that I'm aware of, especially not allowing enough power to make the transmission that you are trying to do. What licensed band are you planning on using, and what is the FCC callsign that you have been assigned/purchased?

David lopez
no, i meant, could you tell me if i use a matching network, would that increase the efficiency of a 3 foot antenna, to the point where i could receive temperature readings, clear video and clear audio at 30 megahertz?

Staff Emeritus
Did you not understand what @berkeman said? Without a license, your planned transmitter would be illegal.

PF will not assist in law breaking. So I am closing this thread. If you have a way to do it legally, click on my user name and start a conversation, then tell me how you will do it legally. If that is convincing, I will reopen the thread.

davenn and berkeman