Omni Pancake VS Donut - which is best at 60feet

Being new to RF I have read almost every post in every WISP forum / list over the last year or so. I still find I do not know near as much as I would like to and next to nothing about what I think should be simple things.

Since the area I want to cover is heavily forested I decided to go with 900MHz. Even in the cities, the largest one having a population of 2,000, almost every house is surrounded by 40ft trees.

So , 900MHz was a given. Next the entire area is hills and valleys and river bottoms. We do have more than a few cellphone towers so I could co locate on them possibly if I wanted to pay a handsome fee each and every month. No, I decided I will try to work with the cities and water districts and maybe trade broadband for access to the water towers.

So, I decided on 900MHz on 60-100ft water towers. Before I was even aware that sectors probably would not work effectively at only 60-100ft off the ground I had decided the small areas that would be covered, due to terrain, did not merit 3+ radios for 600’ish subscribers. Epically when it was unlikely there would be 600 people in any given area (and less likely that ALL of them would sign up for service even if there was).

So, I decided on Omni antennas. Aside from trying to work out the best way place to put the Omni APs throughout the county and keep them from interfering with each other I thought I had everything else worked out.

900MHz Cyclone Horizontal Omnis.

Then I’m googleing around and discover that "High gain omni-directional antennas emit a signal field much flatter than a normal doughnut and are much more like a pancake with a hole in it. A very high gain omni-directional antenna can overshoot clients depending on antenna height and other factors"

Ok, now I’m a bit worried. The Cyclones are only 9db , is that High Gain for a Omni ? They are also horizontal instead of vertical and the do not have antenna patters on the website that I can find so I have no idea if they are Pancake , Donut or Splattering Bacon Grease.

I don’t think Pancake will work for me. My first water tower, if all goes as planned, is on higher ground than most of the surrounding area and then it is 66ft tall on top of that. I’m afraid a pancake is going to be 50 to 60ft over the heads of most of my potential subscribers. At the same time I have no idea how big the ’ donut ’ gets vertically. Will the donut reach the ground 1 mile out, or 6 miles out from the AP ? And what exactly does a Horizontal 9db Omni pattern look like ?

If anyone here uses and understands Omni antennas I would really be grateful for any knowledge you are willing to throw my way on the subject.

so I have no idea if they are Pancake , Donut or Splattering Bacon Grease.

As Larry the Cable Guy would say: "I don't care where you're from, that's funny...."

The spec on that antenna is 15 degrees vertical.

Antennas are measured at their -3dB down (or F3) point. This says the amount of gain will be -3dB lower at X degrees above and below 0 degrees on-axis (assuming no downtilt).

With your antenna, at 7.5 degrees above and below the 0 degree plane, the amount of gain will be around 6dB. However, this is not a brick wall, i.e. the gain does not drop to zero just below the -3dB point, but it does roll off very rapidly. You can expect to have approximately 3dB of gain at around 10 degrees above and below the 0 degree plane.

While 3dB does not sound like much, at 10 degrees you are going to be pretty close to the tower. I would get up on the tower and determine where -7.5 degrees covers and where -10degrees covers. I suspect you will find that it will be just about right.

Distance is also a factor. For every doubling of the distance from the antenna you will lose 6dB through the air.

CPE antenna gain is also a factor. Up close you will not need as much gain and a 6dB yagi might be just right. Further away you will need more foliage penetration and a 13 or 18dB yagi might be the answer.

While 900 is good, don't expect miracles. The key will be be in installing the CPE high enough that it is not trying to penetrate 2 miles of foliage. Hills with no foliage at 2 miles are a show stopper. Add foliage and it's not even worth trying.

Hope this helps.



I didn’t understand a lot of what you said but after I googled around trying to make sense of it I think enough of what you said managed to stick to something in my brain. It put some of my concerns to rest. Or I think it does.

Using the Radio Link tool in Radio Mobile, after the first 1/2 mile or so it is hard to find a spot that is more than 1 degree off according to the Link Tool’s " Elevation Angle = " . Almost all are under a degree. Any place I could place a SM and be more than a degree off 0 ends up being in a river bottom were they would not have ‘terrain’ los anyway (they would be on the dark side of hill).

So if my limited understanding of Radio Mobile is correct, I’m good on angle as long as I’m not in a river bottom or standing under the AP.

I found and bookmarked several web pages that try to explain how to read radio patterns. I know more than I did.


You got the idea. Pretty much anywhere between 0 and -7.5 degrees (looking straight at the antenna) will give you the best possible performance with a 9dB antenna.

As I mentioned, 900 is good but people tend to have unreasonable expectations when it comes to foliage penetrating capabilities.

FYI, knowing what I know about 900, foliage, and noise, if I were setting up a system in an environment like yours, I would use 4X 90 degree or 3X 120 degree sectors. Several reasons I would do this:

- More transmit gain. A sector antenna can get you 12dB or more of gain. +3dB is like doubling your power.

- More receive gain. The AP will be able to pick up weaker signals.

- Better noise rejection. An Omni can see in all directions, this means it can hear the noise from all directions lowering the effective Signal to Noise ratio (The amount of signal from the SM vs. the amount of noise the AP hears). A sector antenna rejects noise that is outside the coverage pattern improving it’s ability to pick up weaker SM’s.

- Flexibility. The Cyclone concerns me in that it offers little in the way of flexibility. If things don’t work out with the omni on the top of the tower, you are stuck as the Cyclone antenna and radio are integrated. At the least I would go with a 9000APC and a separate H-Pol Omni. If things don’t work out you can use the AP with a different antenna and sell the Omni for at least part of what it cost.

If you are definately going Omni, I’d suggest you look into using the MTI Wireless Edge 11.5dB Omni:
- MT-243003/NH - 902-928 MHz 11.5 dBi Horizontal Pol.

By going from 9dB to 11.5dB you almost double the transmit power. The MTI antenna is only 9.5 degrees vertical (-4.75 degrees below the 0 plane), but based on your analysis of the viewable terrain from the tower it does not sound like it will be a problem.

I have heard that you really want to elevate the Omni off the top of the tower as much as possible (10-15 feet) to minimize reflections fromt he top of the water tower.

Hope this helps.

By the way, if you have not already done so, it is strongly recommended that you do an RF analysis of the area.

The last thing you want to do is buy all this, set it up, and find out that 900MHz is too noisy to use. Happens all the time.


Thanks for the info on the Cyclone. I had not even given thought to the AP not being connectorized. I had just assumed it was. I won’t be getting a Cyclone then.

Something I do not understand yet is the 3db = 2x power you state, and I have seen it many other places. At first I assumed that since higher gain antennas radiate a narrower path that it followed logically "more power in a narrower beam = more distance / penetration " but in all the calculations I am trying to learn that does not come out in the math.

Take link budget. If I have 26db transmit and a 10db antenna (EIRP 4watts / 36db) going to a receive unit 6 miles away with a -86 sensitivity, my link budget is something like 114 ( or 35.79 if I add in say, 6 miles of free space loss).

If I swap out the 10db antenna with a more powerful 16db antenna then I have to turn my transmit power down to 10db (or that is my understanding here in the US ) to keep my EIRP at the legal 4watts / 36db . When I do that, my link budget of 114 doesn’t change (or the 35.79 with free space loss).

What I get in antenna gain I lose in transmit power. Or do I have some basic concept completely screwed up ? I had assumed the reason higher gain could be better is because the narrower path of higher gain antennas = a smaller Fresnell (but I haven’t got into Fresnell yet so that may be way off / irrelevant because I haven’t found a Fresnell calc that asks what the gain of the antenna is).

My thoughts on a panel antenna thus far is that they would be great if mounted much higher than the SMs because they can be down tilted while an Omni can not. If I could put the AP 250ft or more above the surrounding area I would go with sectors in a heartbeat. I would have frequency diversity and my customers would be shooting upward at more of an angle meaning they was shooting through less tree and more air. But starting out I think I’ll go with a Omni and switch to sectors later if the Omni fails to perform.

The RF analysis I need to look into. So far I have , in many many locations including were the AP sets now, used the Spectrum Analyzer in the SM. My method, since it came with a panel, started out refreshing the tool over and over and over , rotate it a few degrees , refresh over and over and over, shake, rinse repeat until 360 degrees have been covered. After doing that enough times and never seeing any noise, I pretty much stopped until I can find me a small Omni I can attach to the SM so I don’t have to mess with the panel antenna.

Every once in a while when I refresh there will be a spike on a random freq that hits maybe 60db. They appear to be random or something as they are not consistent (rare to see one and it’s never there on the next refresh) , and out of 10 refreshes I ‘might’ see one spike. Being new to this I really do not know how accurate the Spectrum Analyzer is nor the meaning of the occasional, lone, spike. For the most part all freqs are around the -95 to -85 vertical or horizontal.

Is there something more I should do or a link to someplace with more information on the subject ?

RF power is measured logarithmically. Every 3dB is twice (or half) the power. Don’t confuse this with free space loss. A higher gain antenna will have a narrower beam (or a flatter donut) and as such a further throw at the 0 axis.

To calculate the RF level at the SM receiver:

Radio Tx power
- TxCable Loss (assume 2dB)
+ TxAntennaGain
- Freespace Loss
+ RxAntennaGain
- RxCableLoss (assume 2dB)
+ Rx Receiver Sensitivity
= Fade Margin

Example AP at 924MHz with 11dB omni, 5 miles, 13dB Yagi at SM, clear LOS:
Tx EIRP = 28-2+11 = 37 (factor in other losses and it’s close enough to 36dB EIRP)
Freespace Loss @ 5mi = 109dB
Rx Gain = 13dB-2dB+86 = 97

37-109+97 = 25dB Fade margin

Here is a very good link calculator:

So with a margin of 25dB you would have an RF level at the SM receiver of about -61dB This would be a fantastic link. This is the level before any other losses such as ground clutter, trees, etc. Different trees have different losses. Trees with a lot of moisture and/or sap are the worst. Expect loss through one tree to be anywhere from 8 to 20dB.

Something else happens as the signal passes through the foliage, it scatters. So not only do you have less level at the other side of the tree, you have a damaged signal. Canopy is pretty forgiving, but it’s not indestructible.

Free space loss (inverse square law) tells us that if we cut the distance in half, we gain 6dB, so at 2.5 miles the level at the SM receiver would be -55dB. If we double the distance to 10 miles, the level would be -67. I can tell you from experience, this can be applied to the real world.

Doing your RF analysis the way you did gave you an idea of the RF levels at that time. I suggest you look in the morning, and the afternoon on Mon, Weds, Fri, Sat to confirm that there is no periodic RF source that will blindside you later. by the way, set the SM Web Page Auto Update to 3 seconds. This will make the Spectrum Analyzer page refresh every 3 seconds automatically.

Increasing antenna gain and lowering Tx power is useful if you are trying to get a narrower beam or antenna pattern. As the radiation pattern narrows, the antenna becomes less sensitive off-axis so it picks up less ambient noise. This helps the radios receiver to pick up weaker transmitted signals. If the antenna is picking up a lot of ambient noise, it reduces the sensitivity of the radio receiver and thus requires more signal from the transmitting radio to be heard above the noise. This is the S/N ratio, and the higher that number the better your system will perform.

Hope this helps.

So is it correct that “if you are at max transmit power but still below legal EIRP then a higher gain antenna will allow you to double power ever 3 db up to legal EIRP” BUT “if you are at max transmit power AND max legal EIRP with your current antenna gain, then dropping max transmit to go with a higher gain antenna will not double your power per 3db” ? I understand that it will still have the advantages of a narrower signal but the double power / range / penetration , not so much ?

I also noticed you used " 28db " as your transmit power value. Of interest possibly is that while I was googling around a while back for something unrelated to the current topic I ran into several old posts in various forums that stated " even though you can set the Canopy gear to 28db in the radio config it does not actually go above 26db. Even though it says 28db it is really only 26db"

Most of the posts were old, 2004 - 2005 and I wondered it (1) was ever true , and (2) Most importantly was it true now. According to the specs the 900MHz Canopy is 400 mw ( I think ) and I was pretty sure if I was doing the DBw to watt power conversion correctly then 26db works out to 398.## mw while 28db would work out to 600+ mw. The only way I could think of for me to test was :

I logged into the AP and changed the transmit power to 24. After it came back up I ran spectrum analysis from the SM pointing at it 700ft away and noted the frequency levels the AP was using ( 924 center ). The result was steady at about 57db.

I changed the AP to 25db and again ran the analysis. The result was steady at about 55db.

I changed the AP to 26db and again ran the analysis. The result was steady at about 53db.

I changed the AP to 28db and again ran the analysis. The result was steady at about 53db .

Note: that is also the power level the SM registered to the AP at each setting +/- 1

My methods and knowledge make the results questionable at best, at least to me. But it does appear the posts, claiming canopy’s real max is 26db regardless of the setting, are correct.

we just tested this theory & according to our tests, the Canopy 900mhz WILL indeed transmit at 28db output power. we had to downgrade to 8.2.1 in order to input 28db. after re-upgrading to 9.4.2 the setting holds & indeeds transmits more power.

24db op > -51db rec pow lev
26db op > -49db rec pow lev
28db op > -45db rec pow levi

just fyi for you guys out there.


Is it me or are those results backwards? You had better RSSI at 24 than at 26???

"- Flexibility. The Cyclone concerns me in that it offers little in the way of flexibility. If things don’t work out with the omni on the top of the tower, you are stuck as the Cyclone antenna and radio are integrated."

The 900mhz Cyclone’s are actually connectorized.