Physical Antenna Gain vs Antenna Gain Setting

Hello there,

I was just wondering if someone could help me understand something, preferably with sources. I've looked a bit on Google, but most of the results I've gotten back deal with improving the gain of the antenna itself and not the setting.

We use ePMP 1000s, 2000s, and 3000s in our network. Since I started, the configuration "go-to" has been to set Tx power to maximum and then increase antenna gain until EIRP limits are hit. DFS is kind of a tricky bugger as antenna gain is normally always higher than Tx power (in my experience), but DFS is an exception, not the rule. I'm talking about your standard, 5.2 or 5.7 deployment.

Also our "Max Range" is set to 13 miles. I would imagine decreased Tx power would have some sort of effect on this parameter. Or, maybe not, and 23 dBm Tx power is overkill for 13 mile shots? I just don't know.

What does this do to our radios? Are these settings inherently better than setting the antenna gain to match your antenna and then maxing Tx power from there? What's the difference between these 2 configuration set-ups? How does each of these settings affect signals? Does anyone have any experience with this?

The only new APs we're putting up are 3000s; they currently don't have enough connected customers to really test one way or the other, and I'm cautious about testing on production APs. After-all, if it ain't broke, don't fix it.

Even if someone could point me in the right direction for research, that would be appreciated.

Thank you. :)

Hello, here is some information that may help:

- The EIRP limits are going to be different between an AP and SM (higher EIRP for SM, except for DFS channels)

- The EIRP limits are going to be different based off of channel size (higher channels = lower EIRP, except for DFS channels)

- Some bands have different EIRP limits depending on the proximity to the upper/lower edges of the frequency band (i.e. 5.1GHz)

- Integrated radio/antenna devices will go up to maximum EIRP for the frequency band and configuration

The recommended configuration for an AP (assuming connectorized radio) is to enter the frequency, channel size and other RF parameters.  Then enter in the antenna gain of the attached antenna.  This will limit the configurable maximum TX power to the EIRP limit for the band and configuration.  Not entering an antenna gain (or using a lower value than the actual gain) may cause the radio to transmit higher than FCC EIRP for the band/configuration.  This is especially important to follow in DFS bands (5.2GHz and 5.4GHz).

On the SM side, for a connectorized radio, you should also enter in the attached antenna gain (integrated will have a hard set value).  This will limit the maximum TX power to the EIRP limit for the band and configuration.  The SM is going to automatically adjust it's TX power.  However, since the SM typically has a higher EIRP maximum, there is another parameter that comes into play: the SM Receive Target Level (on the AP Radio Configuration page).  This is the desired uplink signal connection into the AP; the SM is going to automatically adjust it's power to meet (or best attempt to meet) this signal level.  The SM's ability to meet this value is dependent on the maximum EIRP for the frequency band and maximum TX power of the SM hardware.  A SM that is close to the AP will, typically, lower it's power to meet this signal level and, a SM that is far away from the AP will increase it's TX power (up to the hardware maximum) to meet this signal level.

Determining the value to be used for the SM Receive Target Level depends on whether you are doing frequency reuse or not.  If not, then the value can be set as high as desired (typically ~25dB above the noise level).  If using frequency reuse, it should be lower (e.g. -60 - -65dBm) to prevent SM uplink transmissions from overshooting the intended AP and cause interference on the back sector.

Here are some reference links:

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The purpose of the antenna gain setting in the radio is to tell the radio what the gain of the antenna is to prevent you (or the radio)  from "accidentally" exceeding legal EIRP for whatever regulatory domain you told it you are in. It also allows the CPE's to auto adjust TX power and prevent them from exceeding legal EIRP.

If an AP is configured to auto select frequencies then it uses whatever you told it the antenna gain was in order to know how much TX power it can use whatever frequency it picks.

If you are setting TX power to max and then adjusting the antenna gain setting then you are doing it backwards and if the antennas you are using actually have more gain than whatever you set it to in the radio you are probably exceeding legal EIRP.

So that setting isn't really a setting as it is more for informing the radio what gain antenna it is connected to.

The range setting is not about power it's Sync / Timing.  Any AP's you have configured to use Sync must all be set to the same max range setting. A shorter range also has some small performance benifit but I'm not sure how noticeable it might be.

 The only relationship distance/power really have to each other is you might turn power down if it is a really short distance and you are trying to prevent self interference (that is also the reason for the Max CPE RSSI setting, it makes the CPEs use no more power than they need to so they aren't blasting RF miles farther than they need to).

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Doesn't antenna gain setting affect radar detections?  I have allways thought that if you have attached say a 15dB antenna, and you put a lower than the actual ap gain in the antenna settings of the ap - lets say 10dB, you are more likely to get false radar detects?  I thought that the radar detection algoritm has a set threshold of when to trigger a detection.  This threshold should (at least in my mind) take into account the antenna gain.  Lets say the threshold to trigger a radar detect is -65dBm EIRP (this follows ETSI or FCC regulations I assume, and I do not know what actual "threshold" is, so I just use -65 as an example). Then a radar signal of lets say -68dBm EIRP hitting the antenna of the AP with an actual antennagain of 15dB, the AP will "see" this signal as being -68dBm + 15dB = -53dBm which should trigger a radar detect.  If the AP has no idea of what gain antenna is used, it will most likely have loads of false radar.  But if we "tell" the ap what gain the antenna is, it can calculate more precisely if this is an actual radar with strong enough signal to trigger a detect.  Using the above, we can see that the EIRP of the detected signal is -68dBm, while the ap "sees" it to be -53dBm because of the antenna gain. If we take the antenna gain setting in the ap into account, the ap will still "see" the signal as -53dBm, but it knows that the gain of the antenna is 15dB (if we have set the parameter correct), so the AP calculates what the actual EIRP of the possible radar signal is by subtracting the set ap gain from the read signal: -53dBm - 15dB = -68dBm - thus not trigger a detect! If we set the ap antenna gain lower than actual gain, lets say 10dB, the story will be different: -53dBm -10dB = -63dBm - thus triggering a detect (which is false in this case)!  If we set the setting to high, there would possibly be to few detects, and we do not follow regulations.

Please correct me if I'm wrong, as I have allways thought that this is one of the reasons to have this setting in the ap (as well as tx eirp calculation of course).