I am wanting to build 1/2 wave end fed ground plane independent aerials for
2M and 70cms for use on
touring motorcycles for bike to bike comms.

At the moment we are using 5W hand helds with the helicals, but I was
wanting to experiment with half waves
better sited, to increase the max range possible between the bikes -
groundplanes are not practical. On 70cms
the aerials will be physically smaller but not sure how the range might be
affected / reduced

Has anyone suggestions on band to use and/or designs for suitable aerials,
or a link to a site with details ?

Thanks,

Nick

Hi Nick

Generally speaking a 1/2 wave on 70cm will have 1/9 odd the capture area
on 2m and thus less range for the same RX input levels. This is of
course for an LOS situation. I'll admit I have never compared 2m/70cm in
a mountainous vehicle to vehicle situation. I expect reflection would be
the primary mode. How well it works then depends on the size of the
reflectors! You have probably heard that some pine tree needles are the
exact length to attenuate 70cm signals!

I did some experiments years ago comparing horizontal polarization
mobile to base on 70cm. Quite an interesting outcome. The general figure
of merit was that horiz was better than vert by about 9-12dB once you
went past LOS. The signal also didn't chop around as much ("Picket
fencing") eg a vertical signal may swing 30-40dB but the horizontal only
5-10dB. Horiz antennas are "interesting" on 2/70 though. Mine was a
three leaf clover design about 400mm dia on 70cm.

Most important is height, Get the device out of clutter. I think if I
was doing it I'd get one of those long fiberglass whips and put my
halfwave at the top. Probably go for a simple slim-jim or jpole design.
Dual band designs will of course be a compromise so it may be smart to
just go and buy something. A vertical one and a half wave vertical
radiates really badly exactly where you don't want it! (ie the 2m
antenna on 70)

Also consider using SSB rather than FM. Don't know the figure of merit
there but 12-15dB sounds about right!

Apologies for not being specific to your request.

Cheers BOb VK2YQA

Nick wrote:
I am wanting to build 1/2 wave end fed ground plane independent aerials for
2M and 70cms for use on
touring motorcycles for bike to bike comms.

Hi Bob,

Thanks a lot for your long and detailed reply...

I have actually tried a J-Stick on 70cms attached to the top of the
windscreen and it does seem
to work better than the standard helical on the rig mounted "in the clutter"
at the centre of the handlebars.
However, it was a home-brew lash up and I am not sure if it was optimised,
although seemed to SWR well.

Naturally 2M would seem the obvious choice, but we get a lot of QRM amongst
towns especially, and
have defaulted to the quieter 70cms band. That said we seem to manage 2 or 3
miles OK, but we tour
and sometimes get separated by several miles, or deliberately part for some
reason, and it's this that
we would like to cater for.

Repeaters are not really an option as frequently we are in foreign parts and
don't have the data / coverage maps to decide which would be the best
repeater or the time to stop and
work it all out and program it all in to the rig - Simplex with memories
programmed to low or high power
is the desired way forward. It doesn't need to be dual band - perhaps an
elevated 2M commercial
halfwave aerial, and use CTCSS or DCS to cut out the QRM, is the way
forward.
SSB is not an option with the kit we have and probably too fiddly to use
when mobile.

Thanks again for the tips and info,

Nick

Nick

Something I forgot to mention - don't laugh.

How about a high speed/small wing kite to get the j pole up 20ft or so?
If you get out of range just let the thing stream out behind and above
you for greater range.

No doubt real dangerous but there may be a way to reduce the risks some..

I remember doing some propagation tests for a pager service some years
ago. The difference between a low vehicle mounted 1/2 wave and the same
antenna on a 12ft pole was about 10dB. I think this was about 15 miles
from the TX site. 10dB isn't a huge help once you get over the horizon
or obstructed, but every bit helps. Height is might!

Keep in mind that if you use omni gain antennas on 70cm you will reduce
the sky radiation and thus lose the ability to get high angle
reflections from mountains etc. You also get significant pattern lift if
the antenna goes off vertical. I realize you are talking 1/2 waves but
thought it worth mentioning.

It isn't worth using mobile directive antennas BTW (like a quad or
yagi). Your signal reflects from all places around you. The end effect
seems to be higher peaks but lower average signal level.

I am surprised by your only 2-3 miles range. I use to run mobile to
mobile 10W on 2M FM driving around Sydney. It certainly got flaky in
places but 5-10 miles was more the usable range. Perhaps it was a higher
route I was driving on. Never thought about it much before. Might be
worth doing some modeling.

Enough ranting from me!

Bob

Nick wrote:
Hi Bob,

Thanks a lot for your long and detailed reply...

Bob Bob wrote:
Nick

Something I forgot to mention - don't laugh.

How about a high speed/small wing kite to get the j pole up 20ft or so?
If you get out of range just let the thing stream out behind and above
you for greater range.

The greatest height a car/truck can be on the highways is 13'6". Mount
the ant on a lightweight fiberglass pole with the tip of ant at this max
height?

Regards,
JS

"John Smith I" wrote in message
...
Bob Bob wrote:
Nick

Something I forgot to mention - don't laugh.

How about a high speed/small wing kite to get the j pole up 20ft or so?
If you get out of range just let the thing stream out behind and above
you for greater range.

The greatest height a car/truck can be on the highways is 13'6". Mount
the ant on a lightweight fiberglass pole with the tip of ant at this max
height?

Regards,
JS

Thanks for all the responses and tips .... It has to be practical as well
for use when moving at speeds up
to 100 to 120 mph ( not operating at this time) without breaking off (
German Autobahns) as it won't be
possible to stop, remove and stow it safely when motorway/autobahns are
encountered.. and also not
attract unwanted attention from the authorities who may think it is an
unsafe attachment if too big.

Nick

FMI interest John which country? (Although I guess it would be similar
worldwide)

I use to guy a mobile fiberglass pole for that purpose. Was ecstatic
over the performance until I smashed some low service station lights..

Bob

John Smith I wrote:

The greatest height a car/truck can be on the highways is 13'6". Mount
the ant on a lightweight fiberglass pole with the tip of ant at this max
height?

Ground planes are eminently practical. You have a built-in groundplane
called a bike frame/engine/fenders/wheels that are all electrically
interconnected. Mount a small aluminum bracket with a BNC(f) connector as
high and as close to the mechanical center of the bike that you can (the
handlebars sound like the optimum location) and then into a BNC(m) connector
solder a quarter wave 19" whip made out of brass brazing rod for 2m and
another BNC(m) connector with a 6" long brazing rod whip for 440.

You will be amazed at the performance gain over the rubber duckie lashup you
are using now.

Jim



"Nick" wrote in message
...


groundplanes are not practical. On 70cms
the aerials will be physically smaller but not sure how the range might be
affected / reduced

On Mon, 28 May 2007 11:10:04 GMT, "Nick" wrotF:

I am wanting to build 1/2 wave end fed ground plane independent aerials for
2M and 70cms for use on
touring motorcycles for bike to bike comms.

At the moment we are using 5W hand helds with the helicals, but I was
wanting to experiment with half waves
better sited, to increase the max range possible between the bikes -
groundplanes are not practical. On 70cms
the aerials will be physically smaller but not sure how the range might be
affected / reduced

Has anyone suggestions on band to use and/or designs for suitable aerials,
or a link to a site with details ?

Thanks,

Nick

There are a number of commercial dual band ground plane independent
antennas available. Many of the Comet antennas are supposed to be
ground plane independent.
http://www.cometantenna.com/FAQs.php?ID=1

If you want to build your own, a J-Pole design could probably be
adapted. One of the guys I know uses a home build copy of the Arrow
dual band J-pole on his Jeep wagon.
http://www.arrowantennas.com/inst/ijpole.html

Mike kc9doa