can I use an inline amplifier with a 18dbi wifi antenna?

Question

I'm trying to pick up a wifi signal. I have a 14dbi antenna (30ft cable) and I get a signal, but it drops consistently. I purchased a 18dbi Yagi antenna and I hope that will work. But I want to know if I use a inline cable amplifier (around 200mW) if that adds to the antenna strength, or does it just amplifies what signal I lost in the 30ft of cable.
My line of sight is not that clear (trees and branches) but I could probably trim some branches off. Will this help?

Answer 1

You have to be careful here, what you are doing *might* require FCC inspection and approval, assuming you're in the U.S. to make sure you're "certified" and not breaking the rules on power output. This is true even though it's in the unlicensed WiFi 2.4GHz and/or 5GHz bands.

You also need to factor in how many dB loss the 30-foot cable has because this does come into play for your overall power budget.

You didn't mention what distance is involved here. Yes, trees and branches will create scattering of the RF signal. Depending on the distance and if this is 802.11a or b/g you should look up "fresnel zone" and make sure you have a clear RF line of sight, with no more than 40% blockage of the first fresnel zone. Yes, trimming branches will help if it's within your control. But the fresnel zone is widest at the center, not on each end (it's shaped like a football, elliptical). Now, if it's only a short distance, like under a few thousand feet, the fresnel zone is not really a concern.

On the power, I can't answer your question exactly because I'm not sure if you're talking 802.11a in 5GHz or 802.11b/g in 2.4GHz, and you haven't said if this is a point to point link or a point to multipoint link. The rules are different for the frequency and the topology. So there are 4 combos here, 2.4GHz PtP, 2.4GHz PtMP, 5GHz PtP and 5GHz PtMP. I assume you're point to point given the directional antenna.

For a 2.4GHz PtMP you are limited to 36dBm (4 watts) EIRP, that's out of the antenna itself. So if you're AP is already putting out, say, 13dBm (20 mW) and you add a 200mW amp (23dBm) and then an 18dBi antenna, guess what, you're up to 13+23+18 which is 54dBm and you're WAY over and could be fined. Since the EIRP can never be over 4 watts, your 200mW amp is probably way too much, definitely too much if your AP is 20mW or higher, because then the 13+23 puts you up to 36dBm right there! There is a 1:1 rule here for every 3dB of antenna gain the radio/amp power must be reduced by 3dB. So with an 18dBi yagi, your max radio+amp output should be no more than 18dBm or 63mW.

Now, if it's 2.4GHz point to point, things change, there is a 3:1 rule, for every additional 3dB antenna gain (above an initial 6dBi) you must reduce the EIRP by 1dB. So with an 18dBi yagi, you max out at 26dBm from the radio+amp, for a total of 44dBm or 25 watts total EIRP and you're still within the limit.

There is another set of rules for 5GHz PtP and PtMP, and it also depends on which UNII band you're using, but I assume you're doing 2.4GHz PtP here, and I gave both 2.4GHz examples above. Re-post another Q if you need the 5GHz rules.