Front to Back Ratio dB

n2rj

Moderator
Staff member
#2
Basically it is the ratio of the signals received from the the front to those received from the rear of the antenna. It is expressed in dB because you are comparing the signals. The reference level is the signal level received to the rear.

Now why is it useful? Well, most commonly if you have mountains, buildings or other objects to the back of the antenna that reflect signals, you could have multipath which causes dropouts on digital signals and ghosts on analog ones. Having a high front to back or f/b ratio allows you to block those signals by turning the back of the antenna to them while the front of the antenna is aimed towards the desired direction. You are literally turning your back to the signals you do not want.

In some antenna designs having a high front to back ratio also may help with forward gain (gain in the direction that the front of the antenna points to) because some of the signal that may otherwise escape out the back of the antenna and be wasted is now reflected and captured by the driven element(s). In the bow tie reflector antennas such as the CM4228A for example this is why some recommend tying both halves of the screen together (in addition to helping with high VHF reception).
 
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Fringe Reception

Super Moderator, Chief Content Editor
Staff member
#3
fletch,

All antennas receive 'some' signals from unwanted directions, their sides (front-to-side-ratio) and their back end (front-to-back-ratio). Here is a link to a website that visually helps explain the concept: the person went to extraordinary measures to block any signals from entering his antenna from any direction other than its front. There are two pages with two examples. I have no clue why the website is tited as it is:
Key in text the normal way

Jim
 

Don_M

DTVUSA Member
#4
fletch,

All antennas receive 'some' signals from unwanted directions, their sides (front-to-side-ratio) and their back end (front-to-back-ratio). Here is a link to a website that visually helps explain the concept: the person went to extraordinary measures to block any signals from entering his antenna from any direction other than its front. There are two pages with two examples. I have no clue why the website is tited as it is:
Key in text the normal way

Jim
Be sure to click the "Page 2 Rev. 2" link at the bottom to see the Garbage Can Antenna on the second page. Priceless!
 

n2rj

Moderator
Staff member
#5
That garbage can antenna is a hoot! It is basically a scaled up version of the wifi pringles can antenna that wardrivers use. I would so like to build one but I'm afraid that my wife would divorce me if I put that up anywhere near our house!
 

Fringe Reception

Super Moderator, Chief Content Editor
Staff member
#6
n2rj,

I posted the Pringles antenna link here months ago and you are spot-on ... but, I didn't make that connection before, since the Pringles can is aluminum foil lined. I thought about sending the 'garbage can antenna' link to my neighbors above me, in case they give me grief about my tiny UHF Yagis in their view.

Jim
 
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Don_M

DTVUSA Member
#7
... I would so like to build one but I'm afraid that my wife would divorce me if I put that up anywhere near our house!
In addition to an extremely low WAF, the thing would certainly cause nightmares among "condo commandos" nationwide.

:becky:
 

n2rj

Moderator
Staff member
#9
In addition to an extremely low WAF, the thing would certainly cause nightmares among "condo commandos" nationwide.

:becky:
As Jim said, It would probably be legal under OTARD rules. lol

No condo commandos here. One reason why I bought the place, so I can have a SAFE place to put up ham radio towers. :D
 

n2rj

Moderator
Staff member
#10
Don,

Wait a minute: I think it would fit within the one meter rule (at least in one plane). LOL!
FCC Fact Sheet on Placement of Antennas

Jim

Disclaimer: Apartment and Condo dwellers, do not try this at home!
The 1 meter rule applies only to sat dishes. TV antennas can be larger than 1m. The only limit is height (12ft above the roof line). Garbage can antenna might pass muster!

Also, if you are in Alaska, there are no limits whatsoever.
 

FOX TV

Contributor
#12
It is simply a measure of how much signal an antenna can receive from the front of the antenna (Desirable) to the amount of signal it receives from the rear of the antenna ( In most cases Undesirable, as these signals from the rear of the antenna can cause signal ghosting or allow reception of multi path signals more easily), and is normally considered an important characteristic of a directional antenna. Front to back ratio would not apply to an omni directional or circular antenna, which is designed to receive signals from every direction, thus it has no front to back ratio.
 

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