Gain on an antenna

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izzyj4

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I'm getting a little confused here with antenna gain. By my understanding and what I have been told is the less gain you have, the wider the focus of the radio wave is, the more the narrower the wave is. Less gain is better for a hilly terrain area versus more gain is used for more of a flat terrain area.

My question is how much is too much and how little is too little? Also how does gain affect receiving for ham radios and scanners?

I have the basic idea but I think I'm confusing myself.
 

Nightshade

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My question is how much is too much and how little is too little? Also how does gain affect receiving for ham radios and scanners?
Gain can be thought of shouting verses whispering. If to much gain the signal will overwhelm the front end
of the receiver and mush out the transceiver. To little gain will wipe out the signal to the point of disappearing both ways.

This is a gross oversimplification but easy to remember.
 

prcguy

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Not sure if shouting is a good way to relate antenna gain to anything and the OP described some of the attributes of antenna gain in the first post.

Antenna gain is basically taking signals radiated in directions you don't need into directions you want. For an omni this would be concentrating more signal at the horizon and for a Yagi/log periodic, etc it would be concentrating the signal in mostly one direction.

Think of how dim your car headlights would look at 100yds if you removed the bulb from the parabolic reflector which gives the headlight tremendous gain.

Matching the antenna gain to a radio is nonsense, moving your car a few feet will change the signal level at the receiver way more than changing between a high gain antenna and a lousy antenna.

It is true that a simple 1/4 wave whip will usually work better up against hills or mountains with repeaters on top but its not a night and day difference. I'm surrounded by 8,000ft mountains with repeaters and I notice a difference switching antennas but at worst its a little scratchy right up against the mountains with a high gain antenna compared to a 1/4 wave whip. If the high gain antenna completely fades out close to the mountain then the 1/4 whip is probably not usable either.

On the other hand I notice a slight improvement switching to a higher gain antenna at a great distance from the repeaters and I generally use higher gain VHF/UHF antennas since I don't spend my life up against the mountains.
prcguy


Gain can be thought of shouting verses whispering. If to much gain the signal will overwhelm the front end
of the receiver and mush out the transceiver. To little gain will wipe out the signal to the point of disappearing both ways.

This is a gross oversimplification but easy to remember.
 

Nightshade

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Joined
Feb 8, 2010
Messages
30
Not sure if shouting is a good way to relate antenna gain to anything and the OP described some of the attributes of antenna gain in the first post.

Antenna gain is basically taking signals radiated in directions you don't need into directions you want. For an omni this would be concentrating more signal at the horizon and for a Yagi/log periodic, etc it would be concentrating the signal in mostly one direction.

Think of how dim your car headlights would look at 100yds if you removed the bulb from the parabolic reflector which gives the headlight tremendous gain.

Matching the antenna gain to a radio is nonsense, moving your car a few feet will change the signal level at the receiver way more than changing between a high gain antenna and a lousy antenna.

It is true that a simple 1/4 wave whip will usually work better up against hills or mountains with repeaters on top but its not a night and day difference. I'm surrounded by 8,000ft mountains with repeaters and I notice a difference switching antennas but at worst its a little scratchy right up against the mountains with a high gain antenna compared to a 1/4 wave whip. If the high gain antenna completely fades out close to the mountain then the 1/4 whip is probably not usable either.

On the other hand I notice a slight improvement switching to a higher gain antenna at a great distance from the repeaters and I generally use higher gain VHF/UHF antennas since I don't spend my life up against the mountains.
prcguy
I said it was a "gross oversimplification" but was easy to remember without getting all techie about it.
 

AK9R

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Gain is relative, so you have to understand the reference against which the gain is measured. Also, gain and antenna patterns tend to go hand in hand in that one affects the other.

Antenna gain is measured against a theoretical antenna referred to as an isotropic radiator. Imagine an antenna that consists of a single point which radiates equally in all directions. As a result, the antenna pattern is a sphere with the antenna, remember that it's a single point, at the center of the sphere. That's an isotropic radiator. But this is just theoretical--you can't really build an isotropic radiator.

The most basic antenna that you can build is a dipole which consists of two elements, each a quarter wavelength long, placed end to end and fed in the middle. If you oriented your dipole vertically and tested the antenna pattern, you'd find that it's a horizontal dough-nut with the antenna running through the center of the dough-nut. Remember the isotropic radiator? It's pattern is a sphere, but your dipole's pattern is a dough-nut. Because the pattern's different, your dipole antenna has gain, about 2.14 dBi, relative to the isotropic radiator. But you only get that gain in the horizontal plane of the dough-nut. If you measure from directly above or below the antenna, you'd find nearly zero gain. By changing the pattern, relative to the isotropic radiator, you have also gotten some gain, relative to the isotropic radiator.

Dipoles are not real practical for use on a vehicle, so quarter-wave antennas were developed. They only work if they have a suitable ground plane below the antenna. That's because your quarter-wave antenna's element becomes one element of the dipole and the ground plane becomes the other element. Your antenna pattern is now very similar to the dipole, but the dough-nut has been sliced horizontally by the ground plane. And your gain out at the edges of the dough-nut is the same as if you were using a dipole.

As you lengthen your quarter-wave vertical, you'll find a few more critical lengths where the antenna is resonant. Some of those resonant lengths are half-wave, 5/8's wave, and 3/4 wave. You can even stack resonant lengths end to end. (Note that each of those lengths typically require some sort of matching transformer in order to bring the feedpoint impedance in line with the 50 ohms that your antenna is expecting.) Each time you hit one of those resonant lengths, you'll find that your antenna has more gain. But you get that gain by flattening out the dough-nut.

So, your comment about lower gain antennas working better in hilly terrain has some theoretical merit. Imagine yourself at the bottom of a valley and the radio station or repeater you want to reach is at the top of a mountain next to the valley. If you had a high gain antenna, it's pattern will be flat out to the horizon. More of the energy from your radio will be radiated into the side of the mountain instead of up to the top of the mountain. If you had a low gain antenna, your pattern will be more like a puffed-out dough-nut which means more of the energy from your radio will reach the mountaintop.
 
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lmrtek

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I NEVER use more than 6DBD of antenna gain on ANY repeater.
More gain just causes massive drop outs on windy days and huge dead spots at steep look angles.
For the mobile the SAME applies.
In hilly terrain. a 1/4 wave whip can easily outperform a 5/8wave
I use nothing but 1/4 wave whips for police and fire in hilly areas
For my customers in more flat terrain, they get a MAXIMUM of 3db gain 5/8waves.
Big gain usually means big dead spots.

Every time a storm blows in with 35- 55 mph winds, EVERY local repeater with big gain antennas
have major problems talking to their units because the slightest movement causes MASSIVE pattern shifts.

My repeater patterns using DB-222s and DB-408s have virtually no impact from the winds.

A side mounted DB-224 also works well with a side support strut.

Top mounted 6db station masters and DB-224s or DB-420s whip around far too much in the wind and therefore are a poor choice
if you have LIVES depending on the repeater coverage.

You can't compare mobile antennas while stationary, so try driving around with a 1/4 wave for a few weeks noting the range and where the dead spots are
the try a 5/8 wave for a few weeks on the same route.

You will soon see which works better for your situation.

I've been engineering repeater antenna systems for over 30 years so I just know what works in what terrain around here.
 
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prcguy

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Around here we use 10dBD Telewave UHF sticks and have used a few DB-420 9.8dB arrays on 5,000ft mountains and never had the slightest hint of fading due to antennas moving in the wind. The Telewave antennas were ordered with downtilt and the 420s were used so I don't know if they had downtilt.

I and most everyone on our systems runs a gain type mobile antenna, 5/8 ove 5/8 or similar and some users drive along the hills and mountains with no fading. As I mentioned earlier, switching from my UHF gain antenna to a 1/4 spike after finding a bad spot up against the mountains brings in the signal slightly better, but not to the point where one antenna will give a full quieting signal where the other is not useable.

I think maybe you should see why your antennas are moving so much in the wind and causing problems. A Telewave, Superstationmaster or other 20ft long 10dB stick does not bend very easily even in 60-80mph winds and the same goes for the all aluminum DB-420.

I too have been doing this for about 30yrs and I would bet our mountains are a lot taller here in So. Cal compared to Pennsylvania. We should be more sensitive to the problem you described but it just doesn't seem to be a problem here.
prcguy


I NEVER use more than 6DBD of antenna gain on ANY repeater.
More gain just causes massive drop outs on windy days and huge dead spots at steep look angles.
For the mobile the SAME applies.
In hilly terrain. a 1/4 wave whip can easily outperform a 5/8wave
I use nothing but 1/4 wave whips for police and fire in hilly areas
For my customers in more flat terrain, they get a MAXIMUM of 3db gain 5/8waves.
Big gain usually means big dead spots.

Every time a storm blows in with 35- 55 mph winds, EVERY local repeater with big gain antennas
have major problems talking to their units because the slightest movement causes MASSIVE pattern shifts.

My repeater patterns using DB-222s and DB-408s have virtually no impact from the winds.

A side mounted DB-224 also works well with a side support strut.

Top mounted 6db station masters and DB-224s or DB-420s whip around far too much in the wind and therefore are a poor choice
if you have LIVES depending on the repeater coverage.

You can't compare mobile antennas while stationary, so try driving around with a 1/4 wave for a few weeks noting the range and where the dead spots are
the try a 5/8 wave for a few weeks on the same route.

You will soon see which works better for your situation.

I've been engineering repeater antenna systems for over 30 years so I just know what works in what terrain around here.
 
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