Antenna Formula for Gain

[S1deband1t]

I was just wondering if there is a formula to figure out (or estimate) how efficient my antenna is performing over a standard Unity gain antenna rated at 0.0dB. I am using a Wilson 1000 Magnet Mount (for a mobile) placed on a metal plate on my roof. The roof is sloped but I have added 2 metal supports which counter the 35 degree slope of my roof. I have the option of going with a dipole and a permement structure but opt for this for easy takedown and setup. Plus I can always use this antenna as a backup for my vehicle.

Things I have factored in include:

Antenna Type : Magnet Mount, base loaded, 10.5 meter
Antenna Wavelength : 1/4 wave (58")
Gain: +58% over unity gain (on 27.405 mhz)
Height: top of antenna is 26' above ground level
Orientation: level (as opposed to being askew or angled)
Base Plane: 24"x24" Metal Pad (cabinet door about 3/8" thick, no rust)
SWR: 1.0:1

Gain = .90 (Magnet mount vs. straight whip permamently placed) x .90 (1/4 wave coil vs. 5/8 whip) x 1.58 (additional gain vs. standard K40 magnet mount) x 12(?) (for 26' height as estimated as best I could off of Gordon West's equation for a 18' antenna for VFH marine antenna having a dB gain of 9) x 1.3 (Orientation level as opposed to full unity antenna as a sphere) x 1 (for the base plate....i have no idea what this # should be) x 1 (SWR rating)

= 19.96 dB gain

Sounds kind of high to me but I don't know how else to measeure.

BTW: please don't rant at me if this is in the wrong forum. I haven't seen any forums to add this to.

UPDATE: I forgot to add wattage in this equation. Also, Mr. Gordon West's equation was based off a higher frequency 156 mhz VHF rather than
10 1/2 meters at 27.4 mhz. No idea how that will factor in but I took a stab.

 

[prcguy]

I see several things that don't add up here. First, an antenna shortened from a full quarter wavelength (around 108" at 27MHz) will have less than unity gain, even if the unity gain figure is based on dBi instead of dBD. The antenna industry typically uses a full size 1/4 wave antenna over a ground plane as a "unity" reference.

Full size 1/4 wave or 5/8 wave antennas can be compared together and the 5/8 wave will have slightly more gain than the 1/4 wave. In many cases, a shortened 5/8 wave will have less gain than a similar size shortened 1/4 wave. When you shorten up an antenna the percent of a wavelength advantage (if any) goes out the window and a short antenna with more coil or winding's around a fiberglass rod will usually incur more losses than one with less coil or winding's for the same overall size and frequency.

Adding of the gains or losses in this post are in a combination of % and dB, these are unrelated and cannot be added together. The Decibel is a logarithmic function and % must be converted to dB before adding them up.

For relating to dB gain, if an antenna will exactly double your signal strength then it has 3dB gain. If it gives you four times the signal increase then it has 6dB gain. If it increases the signal by 10X then it has 10dB gain. 20dB gain is an increase of 100, 30dB is 1,000 times, 40dB is 10,000 times and so on.

To find out how many dB gain or loss something has, take the rated multiplication of the item, like 8X for an example, enter 8 into your calculator, press LOG, then multiply that by 10. An antenna or amplifier that increases a signal by 8X would then have 9.03dB gain. To reverse from dB gain or loss to ?X, enter the gain or loss like 7.25 for 7.25dB gain, divide by 10 then hit the anti LOG button. 7.25dB gain would then be a signal multiplication of 5.3X.

So, your figure of 58% gain over unity for the Wilson 1000 would be a 3.8dB increase, which for a 58" CB antenna is believable if that's how far down or loss it is from a full size 1/4 wave, but its impossible to have that gain over a full size 1/4 wave or "unity" gain antenna.

My guess of your specific antenna gain is more like -3 to -6dBD gain for the shortened Wilson 1000 over a perfect and large ground plane. Subtract a little bit for the mag mount and its inadequate coupling to the ground plane. Subtract another 3 to 6dB for the totally inadequate 2ft square ground plane. Antenna height on a tall hill sloping away from the desired station will give you some gain, but I don't see that here.

Bottom line, your Wilson mag mount on a 2ft square plate is probably 6 to 12dB down from a full size vertical half wave dipole (dBD) or maybe 4 to 10dB down from a isotropic reference (dBi) or full size 1/4 wave ground plane with four full 1/4 wave radials.

Finally. I would never trust your information from Gordon West. Gordo is a seemingly nice guy and I give him lots of credit for getting countless people ham radio licenses. Otherwise Gordo is not a very technical person and much of his antenna figures come from advertising, which is riddled with lies and false gain claims. Like Gordo's 9dB gain omni VHF marine antenna, which does not exist in dBD or dBi. That comes from an old Shakespeare claim of a roughly 5dB gain antenna that they stretched to 9dB with smoke and mirrors. They might as wall as said 52dB gain because its all a dream.
prcguy

I was just wondering if there is a formula to figure out (or estimate) how efficient my antenna is performing over a standard Unity gain antenna rated at 0.0dB. I am using a Wilson 1000 Magnet Mount (for a mobile) placed on a metal plate on my roof. The roof is sloped but I have added 2 metal supports which counter the 35 degree slope of my roof. I have the option of going with a dipole and a permement structure but opt for this for easy takedown and setup. Plus I can always use this antenna as a backup for my vehicle.

Things I have factored in include:

Antenna Type : Magnet Mount, base loaded, 10.5 meter
Antenna Wavelength : 1/4 wave (58")
Gain: +58% over unity gain (on 27.405 mhz)
Height: top of antenna is 26' above ground level
Orientation: level (as opposed to being askew or angled)
Base Plane: 24"x24" Metal Pad (cabinet door about 3/8" thick, no rust)
SWR: 1.0:1

Gain = .90 (Magnet mount vs. straight whip permamently placed) x .90 (1/4 wave coil vs. 5/8 whip) x 1.58 (additional gain vs. standard K40 magnet mount) x 12(?) (for 26' height as estimated as best I could off of Gordon West's equation for a 18' antenna for VFH marine antenna having a dB gain of 9) x 1.3 (Orientation level as opposed to full unity antenna as a sphere) x 1 (for the base plate....i have no idea what this # should be) x 1 (SWR rating)

= 19.96 dB gain

Sounds kind of high to me but I don't know how else to measeure.

BTW: please don't rant at me if this is in the wrong forum. I haven't seen any forums to add this to.

UPDATE: I forgot to add wattage in this equation. Also, Mr. Gordon West's equation was based off a higher frequency 156 mhz VHF rather than
10 1/2 meters at 27.4 mhz. No idea how that will factor in but I took a stab.

 

[kandrey89]

3dB = 1.413x
6dB = 1.995x
10^(dB/20.) = gain
dB=20*log10(gain)

 

[prcguy]

Your using a 20 log function, which would be for voltage, not power. Power is a 10 log function and used for antenna gain, amplifier, attenuation comparisons, etc. A power gain of 3dB is 1.995X and 6dB is 3.981X, not 1.995X.
prcguy

3dB = 1.413x
3dB = 1.413x
6dB = 1.995x
10^(dB/20.) = gain
dB=20*log10(gain)

 

[KK4JI]

3dB is 2x
6dB is 4x
9dB is 8x
10dB is 10x

Those are the easy ones to remember.

A 1/4 wave on 27.4 MHz is roughly 102". You're running what I believe is a bottom-loaded shortened antenna, so no, it won't have "gain", and certainly not ~20dB. A real 5/8ths wave vertical ground plane antenna will have a little gain (towards the horizon, if mounted properly with no close obstructions) over its 1/4 wave sibling, and and the true 1/4 wave will have some gain over your shortened whip.

EDIT: And FYI, a real 5/8ths wave antenna on 27 MHz is about 22' tall.

 

[cmdrwill]

3dB is 2x
6dB is 4x
9dB is 8x
10dB is 10x

Those are the easy ones to remember.

A 1/4 wave on 27.4 MHz is roughly 102". You're running what I believe is a bottom-loaded shortened antenna, so no, it won't have "gain", and certainly not ~20dB. A real 5/8ths wave vertical ground plane antenna will have a little gain (towards the horizon, if mounted properly with no close obstructions) over its 1/4 wave sibling, and and the true 1/4 wave will have some gain over your shortened whip.

EDIT: And FYI, a real 5/8ths wave antenna on 27 MHz is about 22' tall.

And all that depends on a full groundplane. Most if not all mobile antenna installations on 27 mhz do not have enough groundplane.
And the groundplane IS part of an antenna system. So all the 'gain' goes out the window. Oh went out the window.

 

[KK4JI]

And all that depends on a full groundplane. Most if not all mobile antenna installations on 27 mhz do not have enough groundplane.
And the groundplane IS part of an antenna system. So all the 'gain' goes out the window. Oh went out the window.

Hence my parenthetical statement: "towards the horizon, if mounted properly with no close obstructions".