Hi,

I am looking for recommendation for a design of antenna to operate
in the band 915-928 MHz.

The desired response is omni-directional with gain greater than or equal
to a 1/2 wave dipole, something that does not require a ground plane
and is reasonably straight forward to manufacture in-house.

So far I am considering a 1/2 wave vertical dipole with feed through
lower element, J-Pole and Collinear.

Thanks in advance for any ideas or link to a web site that might have
some homebrew versions I could replicate and try.


--

Kind Regards

David Huisman

David

You are going to have to be more specific as to your intended use.

Assuming you are talking about being omnidirectional horizontally from
the antenna, virtually any form of collinear construction would suit.
This is provided the gain doesnt get so high and the vertical half power
beamwidth doesnt get so narrow that your use of it drops out of the
pattern. (These gain and beamwidth figures being inversely related)

For ease of manufacture I'd probably go for a centrally fed collinear
array with a number of 1/4 wave phasing sections. I'd feed it at the
centre phasing section in a similar manner to a jpole but with a 4:1
coaxial balun. You could make this antenna from (say) a single 3/16"
copper pipe/wire length. This construction may not suit if you want to
run it indoors but will be well suited to side tower mounting.

A "super" jpole or one with a phasing section and extra half wave on top
might be another option. Part the way through this page;

http://www.cebik.com/vhf/jp4.html

Something I havent proven to myself so please take this with a grain of
salt, when feeding a vertical collinear antenna from its base very good
feedline decoupling is needed to reduce the effect of skying the pattern
from the antenna . What is more of a problem though (and what I havent
researched) is that without allowing for power loss (through phasing
delays etc) from one element to the next means you have the same
radiation skying effect, the lower elements radiating more than the
upper. One trick here is to mount then antenna inverted or complicate
the construction with a centered feedpoint.

I havent checked websites for 900MHz plans but I have heard that they
are very popular. It may even be cheaper for you to purchase two medium
gain commercial products (that required a groundplane) and arrange them
like a dipole, feeding them through a 1/4 wave Q section. If an antenna
is designed to be 50 ohms resistive over a ground it will be close to
100 ohms in dipole mode. (Making the Q section 75 ohms will do the trick)

Hope this is helpful

Cheers Bob VK2YQA

David wrote:
The desired response is omni-directional with gain greater than or equal
to a 1/2 wave dipole, something that does not require a ground plane
and is reasonably straight forward to manufacture in-house.

In article ,
David wrote:

Hi,

I am looking for recommendation for a design of antenna to operate
in the band 915-928 MHz.

The desired response is omni-directional with gain greater than or equal
to a 1/2 wave dipole, something that does not require a ground plane
and is reasonably straight forward to manufacture in-house.

So far I am considering a 1/2 wave vertical dipole with feed through
lower element, J-Pole and Collinear.

Thanks in advance for any ideas or link to a web site that might have
some homebrew versions I could replicate and try.

While they are into commercial sales, you could look at what Hyperlink
is selling
http://www.hyperlinktech.com/web/antennas_900.php

--
--------------------------------------------------------
Personal e-mail is the n7bsn but at amsat.org
This posting address is a spam-trap and seldom read
RV and Camping FAQ can be found at
http://www.ralphandellen.us/rv

Bob,

Thanks for the response.

The first is for a poultry house monitoring system that consists of
several "sheds" that may be several km away from the main house.
In addition to coms between the sheds and house, there are links between
sheds (hence require omni directional).

The antenna will be mounted no higher than 3km from the ground and
generally the ground between the antennas is flat.(some installations
may have mild undulations)

If I were to scale up the dimensions of the Super J-Pole shown on the
site, would I need to scale the element diameter too ? This would make
the element diameter less than 1mm at 915 MHz ?

Secondly, is it normal to make the 150MHz version and test it then scale
it up based on optimized dimensions from the lower frequency test ?

Regards

David Huisman


Bob Bob wrote:
David

You are going to have to be more specific as to your intended use.

Assuming you are talking about being omnidirectional horizontally from
the antenna, virtually any form of collinear construction would suit.
This is provided the gain doesnt get so high and the vertical half power
beamwidth doesnt get so narrow that your use of it drops out of the
pattern. (These gain and beamwidth figures being inversely related)

For ease of manufacture I'd probably go for a centrally fed collinear
array with a number of 1/4 wave phasing sections. I'd feed it at the
centre phasing section in a similar manner to a jpole but with a 4:1
coaxial balun. You could make this antenna from (say) a single 3/16"
copper pipe/wire length. This construction may not suit if you want to
run it indoors but will be well suited to side tower mounting.

A "super" jpole or one with a phasing section and extra half wave on top
might be another option. Part the way through this page;

http://www.cebik.com/vhf/jp4.html

Something I havent proven to myself so please take this with a grain of
salt, when feeding a vertical collinear antenna from its base very good
feedline decoupling is needed to reduce the effect of skying the pattern
from the antenna . What is more of a problem though (and what I havent
researched) is that without allowing for power loss (through phasing
delays etc) from one element to the next means you have the same
radiation skying effect, the lower elements radiating more than the
upper. One trick here is to mount then antenna inverted or complicate
the construction with a centered feedpoint.

I havent checked websites for 900MHz plans but I have heard that they
are very popular. It may even be cheaper for you to purchase two medium
gain commercial products (that required a groundplane) and arrange them
like a dipole, feeding them through a 1/4 wave Q section. If an antenna
is designed to be 50 ohms resistive over a ground it will be close to
100 ohms in dipole mode. (Making the Q section 75 ohms will do the trick)

Hope this is helpful

Cheers Bob VK2YQA

David wrote:

The desired response is omni-directional with gain greater than or
equal to a 1/2 wave dipole, something that does not require a ground
plane
and is reasonably straight forward to manufacture in-house.

--

Kind Regards

David Huisman
General Manager
-----------------------------------------------------------------------
ORBIT COMMUNICATIONS Pty Ltd - Wireless Solutions that Work
(Telemetry, Control, Monitoring, Security, HVAC ...)
A.C.N. 107 441 869


Website : http://www.orbitcoms.com
PO Box 4474 Lakehaven
NSW 2263, AUSTRALIA
Phone: 61-2-4393-3627
Fax : 61-2-4393-3685
Mobile: 61-413-715-986

Hi David

Okay so you can afford to use a relatively high gain system. You also
need to have the antennas high and clear as you can assume that 900MHz
is line of sight. You could model the exact environment if you have non
LOS paths. That will indicate whether it will work or not. Changes in
antenna gain for an omni probably wont help much if the link is marginal
when modelled. You could also establish whether you could (say) do the
whole system with simple 1/4 wave whips! (A 6dB increase in power or
combined antenna gain over an LOS path gives you twice the range)

If you are scaling an antenna yes you need to scale everything. I didnt
suggest you scale the Cebik antenna, just wanted to show you the
possibilities. It might however be more use to build an adjustable one
and have some method of recording/measuring the field strength and
changing dimensions for a maximum. I wouldnt call the super J pole an
especially critical design for dimensions though. You could for example
use 1/8" brazing rod for elements and make the length marginally shorter
by the amount indicated in end effect tables. The more elements you have
the more critical the dimensions and adjustment will become.

I would say it isnt normal to build at 150 for 900MHz. Too many
variables to account for.

What is on 900MHZ? Is this a data network like 2.4GHz 802.11?? Will
multipathing be an issue? (High data rates usually need directive
antennas not only for a satisfactory S/N with gain, but also to reject
any possble multipathing problem of a data bit arriving later than it
should.)

Cheers Bob

David wrote:
If I were to scale up the dimensions of the Super J-Pole shown on the
site, would I need to scale the element diameter too ? This would make
the element diameter less than 1mm at 915 MHz ?

Secondly, is it normal to make the 150MHz version and test it then scale
it up based on optimized dimensions from the lower frequency test ?

David wrote:
"In addition to coms between the sheds and house, there are links
between sheds (hence require omni-directional)."

The benefit of simple and directional 915 MHz antennas is so significant
that you might consider making one site a repeater and all other sites
directional, communicating through the repeater. A small antenna can
have much gain at 915 MHz. You could conveniently use full-duplex (2
frequencies) for simplicity..

Best regards, Richard Harrison, KB5WZI

Richard,
If I was to use directional antennas. What would your recommendation be ?

What about a vertically polarized Yagi ? other ?

Thanks

Regards

David

Richard Harrison wrote:
David wrote:
"In addition to coms between the sheds and house, there are links
between sheds (hence require omni-directional)."

The benefit of simple and directional 915 MHz antennas is so significant
that you might consider making one site a repeater and all other sites
directional, communicating through the repeater. A small antenna can
have much gain at 915 MHz. You could conveniently use full-duplex (2
frequencies) for simplicity..

Best regards, Richard Harrison, KB5WZI


--

Kind Regards

David Huisman
General Manager
-----------------------------------------------------------------------
ORBIT COMMUNICATIONS Pty Ltd - Wireless Solutions that Work
(Telemetry, Control, Monitoring, Security, HVAC ...)
A.C.N. 107 441 869


Website : http://www.orbitcoms.com
PO Box 4474 Lakehaven
NSW 2263, AUSTRALIA
Phone: 61-2-4393-3627
Fax : 61-2-4393-3685
Mobile: 61-413-715-986

Dave wrote:
"What would your recommendation be?"

I would use a vertical collinear omni antenna for the repeater and
vertically polarized Yagis for each of the "sheds". To avoid stimulation
of argument, I`ll say no more. You mainly need a line-of-sight path at
915 MHz between all remote sites and the repeater.

Best regards, Richard Harrison, KB5WZI

Richard Harrison wrote:

Dave wrote:
"What would your recommendation be?"

I would use a vertical collinear omni antenna for the repeater and
vertically polarized Yagis for each of the "sheds". To avoid stimulation
of argument, I`ll say no more. You mainly need a line-of-sight path at
915 MHz between all remote sites and the repeater.

Best regards, Richard Harrison, KB5WZI


Motorola 900 Mhz Canopys are advertised as "non line of site", so you
may get more mileage than you'd expect when running at this relatively
low frequency. If you are looking at paths that can be well defined,
try a program called "Radio Mobile" for a very good simulation of the
situation. It is freeware, and the databases it uses are also free
(courtesy of the US taxpayer and some hardworking US Shuttle astronauts
and groundcrew).

tom
K0TAR

Tom Ring wrote:
"Motorola 900 MHz Canopys are advertised as "non line of sight"... so
you may get more mileage than you`d expect when running at the
relatively low frequency."

It`s true that radio does not always take a direct path. Part of ham
radio fun is in anomalous propagation. It`s also true that the higher
the frequency, the smaller the obstruction that can block propagation.

900 MHz is more reliable than frequencies in the GHz. A point to
multipoint system may require more reliability than a single point to
point system, and more reliability than: "Can You believe it? I`ve
contacted DX!" I`ve engineered and installed several 900 MHz systems
which proved to provide, with their path clearances and fade margins,
24-7 reliability of very nearly 100%.

Line of sight is defined as: "The distance to the horizon from an
elevated point, including the effects of atmospheric refraction."
Another definition is: "The propagation characteristic of microwave
radio."

900 MHz is on the border between UHF and microwaves. It shares some of
the characteristics of both.

Best regards, Richard Harrison, KB5WZI