Cruisers Forum
 


Join CruisersForum Today

Reply
 
Thread Tools Rating: Thread Rating: 3 votes, 3.67 average. Display Modes
Old 20-02-2011, 03:53   #691
Registered User

Join Date: Jul 2008
Posts: 126
Two more points about high gain antennas.

As to maximum legal power. It is true that if you put a high gain antenna on a high power transmitter you can exceed the 36dB limit of Effective Radiated Power (ERP) but with an 8 or 9 dBi antenna that will rarely happen. When two wifi devices connect they negotiate the protocol (b,g or n) based on the capabilities of the two devices and the quality of the connection. The client side then monitors the number of failed packets and adjust the speed and transmit power to keep the fail count to a minimum. As the quality of the link goes up the speed goes up and the power goes down. In a marina the speed on an 802.11g connection might be the full 54 Mbps but the Bullet will drop the output power by 5 or 6 dBm. OTOH, if you are in a distant anchorage and the signal from the access point is weak the Bullet will slow down and increase power to maintain the link. Then you probably will exceed the legal limit a little but you will also probably be the only signal around.

There is such a thing as having to much gain. As the gain goes up the beam pattern narrows. A 15dBi omni will fade as the boat rocks and a 15dBi panel or cantenna fades as the boat changes direction. Also, in combination with a 29dBm transmitter, a 15dBi antenna will always be over the legal limit. An 8 or 9 dBi omni is about the sweet spot for gain, beam pattern and staying legal.

The real advantage of a high gain antenna is on the receive side. Unlike transmitted power, receive sensitivity does not change. An 8dBi antenna will be able to pull in weaker signals than a 5dBi antenna. The link may be slow and the transmit power a dB or 2 higher than the FCC would like but at least it can be maintained.
__________________

__________________
Gashmore is offline   Reply With Quote
Old 20-02-2011, 21:06   #692
Marine Service Provider

Join Date: Jan 2010
Location: Toronto
Boat: Beneteau Oceanis 411
Posts: 72
Send a message via Skype™ to mlahrkamp
Re: Long-Distance WiFi Device

Quote:
Originally Posted by Gashmore View Post
The real advantage of a high gain antenna is on the receive side. Unlike transmitted power, receive sensitivity does not change. An 8dBi antenna will be able to pull in weaker signals than a 5dBi antenna.
What makes you believe that an omni antenna's dBi rating has anything to do with receive gain? it is only a measure of its transmitting characteristics. If in doubt about this, try it. Take a 5 or 6 dBi and test it for receive against an 8 or 9 dBi. if you see any measurable receive difference, it wil be marginal and nowhere near the difference that the antenna's dBi ratings suggest.

If it does show additional hotspots, they will be so weak as to be unusable.

Mike
__________________

__________________
mlahrkamp is offline   Reply With Quote
Old 21-02-2011, 02:46   #693
Registered User

Join Date: Jul 2008
Posts: 126
Re: Long-Distance WiFi Device

LOL, That's easy. I am a ham radio operator, W4WGA. Been building antennas for 40 years. I have so much wire hung in my yard the birds need an escort to fly over it safely.

Antennas are passive devices. While by convention antenna parameters are expressed in terms of transmission, gain is the same in either direction. It is called reciprocity. Things can be added to the feed line that effect the balance but the basic antenna gain is equal for transmitting and receiving.
__________________
Gashmore is offline   Reply With Quote
Old 21-02-2011, 06:22   #694
Registered User
 
richardhula's Avatar

Join Date: Nov 2008
Location: UK
Boat: S M Hudson 60' narrowboat
Posts: 347
Send a message via Skype™ to richardhula
Re: Long-Distance WiFi Device

Quote:
Originally Posted by mlahrkamp View Post
What makes you believe that an omni antenna's dBi rating has anything to do with receive gain? it is only a measure of its transmitting characteristics. If in doubt about this, try it. Take a 5 or 6 dBi and test it for receive against an 8 or 9 dBi. if you see any measurable receive difference, it wil be marginal and nowhere near the difference that the antenna's dBi ratings suggest.

If it does show additional hotspots, they will be so weak as to be unusable.

Mike
Mike the lack of change in "perceived" signal strength is probably being masked by AGC circuitry which is fooling you into making false statements.

As a another radio HAM of 45 years standing I agree with Gashmore.
__________________
Richard

If in doubt RTFM
richardhula is offline   Reply With Quote
Old 21-02-2011, 06:48   #695
Do… or do not
 
s/v Jedi's Avatar

Cruisers Forum Supporter

Join Date: Feb 2009
Location: in paradise
Boat: Sundeer 64
Posts: 9,198
Re: Long-Distance WiFi Device

Quote:
Originally Posted by mlahrkamp View Post
What makes you believe that an omni antenna's dBi rating has anything to do with receive gain? it is only a measure of its transmitting characteristics.
Gashmore is correct here: the gain of an antenna works for both transmit and receive.

Gain from a passive antenna is achieved by focussing the radiation pattern. Besides the higher gain, it also eliminates (attenuates) noise that would be received from the sector(s) outside the radiation pattern. This means that the higher the gain (the more focused), the better the quality of both transmit and receive. The cost is the need to carefully position the antenna so that the "other side" is within the radiation pattern. This is true for both omni-directional and directional antennas. (for omni-directional you get the donut-shaped pattern which gets flatter as gain is increased).

cheers,
Nick.
__________________
s/v Jedi is offline   Reply With Quote
Old 21-02-2011, 07:32   #696
Registered User

Join Date: Apr 2008
Posts: 208
Re: Long-Distance WiFi Device

Quote:
Originally Posted by s/v Jedi View Post
Gashmore is correct here: the gain of an antenna works for both transmit and receive.

Gain from a passive antenna is achieved by focussing the radiation pattern. Besides the higher gain, it also eliminates (attenuates) noise that would be received from the sector(s) outside the radiation pattern. This means that the higher the gain (the more focused), the better the quality of both transmit and receive. The cost is the need to carefully position the antenna so that the "other side" is within the radiation pattern. This is true for both omni-directional and directional antennas. (for omni-directional you get the donut-shaped pattern which gets flatter as gain is increased).

cheers,
Nick.
I'm new to ham radio, (KB3THV) but I have been experimenting with antennas for wifi, including building my own, for several years now and I can tell you from real world experience (although this is considered anecdotal by the engineers) that gaining in the recieve department is actually just as important and desirable for usable wifi as transmit is. Also, a quality antenna is as important to wifi as quality speakers are to a stereo system. You can't just keep pumping power into junk and expect improved results.

In terms of good netizenship. I think it's important enough that the "quick setup" guides floating around could be amended to include at least a foot note about transmit power control.


George

P.S. Thinking back on my military experience, receiver sensitivity in the equipment I was dealing with was considered critical. It was the difference between being able to stop a missile in mid flight if we had to or it actually going rogue and locking onto Mr. Lee's shrimper out in the Gulf of Mexico.
__________________
thatboatguy is offline   Reply With Quote
Old 21-02-2011, 08:06   #697
Registered User

Join Date: Jul 2008
Posts: 126
Re: Long-Distance WiFi Device

Quote:
Originally Posted by thatboatguy View Post
P.S. Thinking back on my military experience, receiver sensitivity in the equipment I was dealing with was considered critical. It was the difference between being able to stop a missile in mid flight if we had to or it actually going rogue and locking onto Mr. Lee's shrimper out in the Gulf of Mexico.
That is why I prefer the Bullet 2HP over the M2HP for the cruisers I help. The the receive sensitivity of the 2HP at 24Mbps is -89dBm in B mode and -86dBm in G mode. The M2HP is optimized for N mode so B and G are limited to less than half that.

If you are cruising in the contenental US N hotspots are becoming more common so the N2HP is probably a good choice. But in the Caribbean a hotspot with N capability is rare. In fact, in the islands serviced by Cable & Wireless it looks like N is being skipped all together in favor of WiMax but that is another game all together. Only advice i can give there is don't spend to much money on your b/g/n gear.
__________________
Gashmore is offline   Reply With Quote
Old 21-02-2011, 08:30   #698
Registered User

Join Date: Apr 2008
Posts: 208
Re: Long-Distance WiFi Device

I put the 2HP at the masthead with the best Omni I could find. That frees up the 8dbi omni that was up there with the M2HP and those become "hoistable". I also have a short coax connector that allows me to hook an Alfa 1 up to that omni for a hoist-able, in case I need to "survey" an area or something. I can also hook the M2HP up to the Cantenna which is cool because the bullet becomes a pistol grip for those times when nothing but a tight beam, hand aimed will get me that email through.
__________________
thatboatguy is offline   Reply With Quote
Old 21-02-2011, 08:38   #699
Registered User
 
richardhula's Avatar

Join Date: Nov 2008
Location: UK
Boat: S M Hudson 60' narrowboat
Posts: 347
Send a message via Skype™ to richardhula
Re: Long-Distance WiFi Device

I think the main differences in using a Bullet 2HP & M2HP (specs aside) are generic & not attributable to the devices as such. 802.11n devices can work in three modes:
greenfield - pure 802.11n, no legacy compatibility
legacy mode - frames are transmitted in the legacy 802.11a/g OFDM format
mixed mode - packets are transmitted with a preamble compatible with the legacy 802.11a/g

Its the latter mode that the present Ubiquiti firmware locks the Bullet M2HP into & gives justification to claims that the 2HP is better with legacy networks.

For the record I'm using an M2HP now connected to an 802.11b AP a half mile away with a very strong -40dBm signal, so the difference is marginal at best.
__________________
Richard

If in doubt RTFM
richardhula is offline   Reply With Quote
Old 21-02-2011, 08:51   #700
Registered User

Join Date: Apr 2008
Posts: 208
Re: Long-Distance WiFi Device

At 4.8 miles away it makes a difference! And here's one to ponder. There is a station at a resort that's only a couple miles away. They are running WEP security which code was given to me by someone who works there. Anyway, we were able to log on with the M2HP and it gave us an IP and everything worked fine. But with the 2HP... no dice. Good strong signal, associated fine, "no IP for you". Weird. If it were "N" then it shouldn't even show up in the available list for the 2HP right?
__________________
thatboatguy is offline   Reply With Quote
Old 21-02-2011, 11:23   #701
Registered User
 
richardhula's Avatar

Join Date: Nov 2008
Location: UK
Boat: S M Hudson 60' narrowboat
Posts: 347
Send a message via Skype™ to richardhula
Re: Long-Distance WiFi Device

Quote:
Originally Posted by thatboatguy View Post
At 4.8 miles away it makes a difference! And here's one to ponder. There is a station at a resort that's only a couple miles away. They are running WEP security which code was given to me by someone who works there. Anyway, we were able to log on with the M2HP and it gave us an IP and everything worked fine. But with the 2HP... no dice. Good strong signal, associated fine, "no IP for you". Weird. If it were "N" then it shouldn't even show up in the available list for the 2HP right?
Wrong - backward compatible both ways unless the 802.11n AP is specifically set in "greenfield" mode.
__________________
Richard

If in doubt RTFM
richardhula is offline   Reply With Quote
Old 21-02-2011, 11:28   #702
Registered User

Join Date: Apr 2008
Posts: 208
Re: Long-Distance WiFi Device

Quote:
Originally Posted by richardhula View Post
Wrong - backward compatible both ways unless the 802.11n AP is specifically set in "greenfield" mode.
Pure curiosity at this point but what could cause the set of circumstances where both the 2HP and the M2HP can "see" and associate with the access point but only the M2HP ends up getting a DHCP lease and an IP with internet. The 2HP tries for a while and then falls back to default.

George
__________________
thatboatguy is offline   Reply With Quote
Old 21-02-2011, 11:38   #703
Registered User
 
richardhula's Avatar

Join Date: Nov 2008
Location: UK
Boat: S M Hudson 60' narrowboat
Posts: 347
Send a message via Skype™ to richardhula
Re: Long-Distance WiFi Device

Can't answer that with confidence George although I am feeling kind of smug that its not the other way around, having an M2HP myself.

Some freek setup thing maybe at your end. A weird configuration on the AP. Different signal strengths between the two devices?

Have you tried connecting with other legacy devices & if so do you get the same issue?
__________________
Richard

If in doubt RTFM
richardhula is offline   Reply With Quote
Old 21-02-2011, 14:23   #704
Registered User

Join Date: Apr 2008
Posts: 208
Re: Long-Distance WiFi Device

I'm closer to the station now and will try with the alfa 1. I was thinking possible configuration issue but one of the heavy hitters here shared his configuration file with me and even that didn't solve the problem. Oh well... If I ever run into this again I can always break out the M2... heh heh

George
__________________
thatboatguy is offline   Reply With Quote
Old 22-02-2011, 19:42   #705
Do… or do not
 
s/v Jedi's Avatar

Cruisers Forum Supporter

Join Date: Feb 2009
Location: in paradise
Boat: Sundeer 64
Posts: 9,198
Re: Long-Distance WiFi Device

This whole thing about the Bullet2HP having a more sensitive receiver than the BulletM2 is a fairytale. I read about values for 36 Mbps speed while it doesn't really matter if it wouldn't work because the link can fall back to 24 or 11 or 1 Mbps and it would work anyway. Also, The M2 has the same sensitivity at 48 Mbps so there it is.

The whole thing is that the M series has a new generation of chips, a CPU that is more than twice as fast and it has n-mode for 20 bucks more. In a couple of months the old radios might be discontinued and we'll all have to buy the M series anyway.

Like somebody else already posted, the older radios will start showing incompatibilities too. The technology is driven by the wifi chip makers, not by companies like Ubiquity.

The Caribbean might be a bit backward of a place for technology but they have FedEx there too and a nice new NanoStationM2 can be had overnight plus an extra day for customs and there you have your n-mode radio. The people running the wifi hotspots in the Caribbean can and will upgrade as soon as one of the competitors gets a new radio. Meanwhile, the M series does pretty good legacy modes.

When you stay in the same area with a good wifi station where you have a subscription and you can work it from whereever you sail with an antenna and Bullet at the masthead, that's just great. But if you sail around you're not gonna scan the band for a station when you sight the next island, do you? Well, I don't and I never heard of others doing it (war sailing?). Situations will vary so wildly that I wouldn't put another antenna at the masthead because in 90% of the cases a nanostation or even picostation at the rail will work just fine and in the 10% left you can easily tie that Bullet to a halyard and just hoist it.

cheers!
Nick.
__________________

__________________
s/v Jedi is offline   Reply With Quote
Reply

Thread Tools
Display Modes Rate This Thread
Rate This Thread:

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Similar Threads
Thread Thread Starter Forum Replies Last Post
Cats Better for Long Distance Sailing? Cavecreature Multihull Sailboats 68 29-05-2009 02:05
Long distance communication MDhillon Navigation 2 10-03-2009 17:39
Long distance transport? dory36 Dollars & Cents 5 25-01-2007 13:35
buying and owning boats long distance capt lar Monohull Sailboats 10 29-03-2005 08:15



Copyright 2002- Social Knowledge, LLC All Rights Reserved.

All times are GMT -7. The time now is 23:08.


Google+
Powered by vBulletin® Version 3.8.8 Beta 1
Copyright ©2000 - 2017, vBulletin Solutions, Inc.
Social Knowledge Networks
Powered by vBulletin® Version 3.8.8 Beta 1
Copyright ©2000 - 2017, vBulletin Solutions, Inc.

ShowCase vBulletin Plugins by Drive Thru Online, Inc.