Force400 F425 Link Quality flapping between 100% and 0%

I hope I am not out of line in re-opening this thread. I have this exact same issue with a Force400 F425. Client is in a rural area and the AP is on a roof-top 140ft in the air on a small island with what seems to be low interference. Link Quality is constantly jumping from 100% to 0%. The Link Capacity also jumps but not as frequently. The client is reading Downlink: RSSI-67~68 dBm SNR29~30 dB. Transmitter Output Power 30 dBm, Antenna Gain 40 dBi (both maxed out). Distance is 6 miles. I’ve tried changing the MCS from 11(max) down to 5 with no change. Also tried both DS and SS. Frequency is 5835 MHz with a 20 MHz bandwidth. I have also tried 40 MHz with no help. I am pushing about 45 Mb/s of video 24 hours a day. Attached is wireless stats form the client.

This is the spectrum on the client side.

This is the spectrum on the AP side.

So first and foremost, something is SERIOUSLY wrong if your TX power is maxed out at 30dBm and you’re using a 40dBi antenna (which can’t possibly be correct because it would be enormous)… and your RSSI is only -67 (LOS @ 6mi). Either your alignment is way way way off, or something is physically wrong with the radios or antennas.

Even if you were using 30dBm TX power and like a 1.5’ dish at 25dBi (e.g. a F425), your EIRP would be 55dBm, which, assuming this is 6mi LOS, should result in an RSSI of around -45dBm on both sides.

You should try modeling your link in Cambium’s free Link Planner to get a better idea of what RSSI, modulation, and throughput to expect.


On the F425 radio DO NOT CHANGE the antenna dBi… leave it at the default of 25dBi.

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That’s not how antenna gain works. That doesn’t turn the gain up or down… that’s simply you telling the radio what is actually attached.

So, if you actually have a 25 dbi antenna attached, that’s what it needs to be set to. If your turn it ‘up’ then the radio thinks it’s got a 40dbi antenna, and it can turn power down to try to compensate… and it will probably screw things up.

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Thanks for the reply. So what’s the max antenna gain on the F425? I see now that the F425 has a max transmit power of 28db but the settings go up to 30db. The settings seems to be misleading as they offer antenna gain and transmit power variables that are not actually available.

  1. The Force 425 has a 25 dBi gain antenna.
  2. There is also an add-on antenna extender, which makes it 28dBi gain.
  3. And there is also the Force 400c (same radio electronically) which connects to external antennas, and they could be 30 dBi or 34 dBi or 40 dBi (or 10 dBi) or whatever you choose to connect.

If you connect some other antenna, that field is where you would ‘input’ and telling the radio what gain the attached antenna is, so it’s not where you ‘set’ the gain exactly.

Well, that is not really a ‘setting’ exactly. It is more where you are telling the radio what type of antenna you have attached. There’s no way a radio can know what you’ve happened to connect. A F425 normally would have a 25dBi antenna, but someone (KP Performance for example) could make their 29 dBi Proline antenna for the F425, in which case you’d want to tell the radio that is what you have connected, by inputting 29 dBi into the gain field.

But separately from all that – if you have F425’s at 6 miles, you should be able to achieve a -50 signal and get DS11 modulations - if it’s all aimed properly, if there are no trees or other obstructions, and if your Fresnel zone is clear. That should work very well. :slight_smile:


Thanks for the info. There are no trees or obstructions although the signal is traveling over part of a bay and a shipping channel. I adjusted the antenna gain down to 25db but I just cant seem to get the signal any louder than -69db ~ -70db. They seemed to be aimed correctly unless there is some tiny micro positioning that I’m missing.

You miss ~25db.
Check again your alignment.