Curiosity has struck me: antenna degradation

[Danny37]

I've been curious about this but I noticed when a radio is clipped to a belt close to the body it doesn't recieve as well. Does our physical body act as a ground when it comes in contact with an antenna?

Also when you touch the antenna on a handheld radio the signal quickly drops as seen via the RSSI indicator and the audio passing through. I just want to know the science behind it.

 

[mmckenna]

Us humans are just bags of meat and water. Neither propagate RF well.

A radio worn on the belt will have some of the RF absorbed by the body, ultimately turning into heat. Some RF will be radiated, but away from the body. It's one of the worst places to carry a radio. The antenna that close to the body probably detunes it to a certain extent. Some "public safety speaker mics" have the antenna remote'd to the top of the mic, which can improve performance.

Wrapping your hand around the antenna sort of does the same thing.

 

[Danny37]

Us humans are just bags of meat and water. Neither propagate RF well.

A radio worn on the belt will have some of the RF absorbed by the body, ultimately turning into heat. Some RF will be radiated, but away from the body. It's one of the worst places to carry a radio. The antenna that close to the body probably detunes it to a certain extent. Some "public safety speaker mics" have the antenna remote'd to the top of the mic, which can improve performance.

Wrapping your hand around the antenna sort of does the same thing.

Is the detunning a temporary occurrence or is it a permanent defect when the attenna comes in contact with the body?

 

[nd5y]

Is the detunning a temporary occurrence or is it a permanent defect when the attenna comes in contact with the body?

It happens the whole time the antenna is in contact or close proximity to you. It won't permanently damage the antenna or change it in any way if that's what you meant.

 

[Danny37]

It happens the whole time the antenna is in contact or close proximity to you. It won't permanently damage the antenna or change it in any way if that's what you meant.

Yup exactly what I meant.

But there's times (really rare) when touching the antenna actually improves reception. Any reason why that might be?

 

[wtp]

an old term for people is "stray capacitance to ground".

 

[mmckenna]

Yup exactly what I meant.

But there's times (really rare) when touching the antenna actually improves reception. Any reason why that might be?

You could be detuning it from it's designed frequency to one that is more proximal to the frequency you are receiving.
You could be making it slightly directional.

I'll assume you don't have any metal plates in your body, tin foil hat, eat rusty nails for breakfast, carrying around shrapnel, are actually a robot wearing someone else's skin, etc...

 

[RFI-EMI-GUY]

Yup exactly what I meant.

But there's times (really rare) when touching the antenna actually improves reception. Any reason why that might be?

When everything is in phase. Really! your body conducts radio waves and at times touching the antenna will impart a signal into the radio, especially if you are not also at same time grounding it. Your body also serves as a counterpoise to return signal to earth, so if you touch the antenna just right, the radio itself becomes the antenna . Its magic really.

The human body is actually resonant around 75 to 100 MHz or so making that region especially sensitive to high RF fields when considering safety around FM broadcast antennas..

 

[RFI-EMI-GUY]

an old term for people is "stray capacitance to ground".

That sounds kind of derogatory like "a waste of oxygen". I like it. "Never mind him, he is just stray capacitance to ground".

 

[mmckenna]

That sounds kind of derogatory like "a waste of oxygen". I like it. "Never mind him, he is just stray capacitance to ground".

Or as I tell my guys at work, "Don't become part of the circuit".

 

[Danny37]

I have most of the guys at my squad using the Motorola "high activity" swivel which points the radio at an angle away from the body which improves reception compared to against the body using the belt clip.

 

[Danny37]

The human body is actually resonant around 75 to 100 MHz or so making that region especially sensitive to high RF fields when considering safety around FM broadcast antennas..

Intresting that you mention that, I notice it affects VHF more compared to UHF in my experience.

On a side note, different antenna have different results. Using Motorola UHF antennas as an example, the standard 6 inch whip seems to have no to very little effect compared to a 3 inch stubby that cuts the signal in half when touched.

 

[RFI-EMI-GUY]

If you ever wondered how very bad those portable antennas are. Here is some research from 1991. I think the work is still valuable today, if anything these new multiband antennas on APX etc, are probably worse.

 

[Rt169Radio]

Good info, always wondered about that.

 

[speedway_navigator]

I'm curious how they design portable antennas for multi band radios. The antenna on the VHF 800 MHz APXs I saw looked to be 7" by about 5/8" diameter.

 

[nd5y]

I'm curious how they design portable antennas for multi band radios. The antenna on the VHF 800 MHz APXs I saw looked to be 7" by about 5/8" diameter.

From what I have read the APX multi band antennas have several elements in them.