Thanks for the feedback everyone.

The article Jerry pointed me to starts looking like the commercial
antenna I was initially studying. Their unit though only had a 1/4 wave
above the coaxial sleeve.

From the SMA plug, the coax coiled around the radome must have been the
choke to help reduce RF currents radiating from the earth braid.
Then the 1/4 wave length coax forms the match followed by the expose
radiating section.

It looks like an open stub fed J-pole where the stub is enclosing the
inner element rather than being constructed as a rod next to it.

Does this seem reasonable ? (PS. I'm a beginner with regards to antenna
theory and would like to understand what is happening so that I can
experiment with some kind degree of success).




Cecil Moore wrote:
David wrote:

Thanks for the info. The open stub J looks like it should be suitable
for my application being ground independent, omni directional and
having low radiation angle.


Be sure to include a choking function at the feedpoint.

In article ,
David wrote:

The article Jerry pointed me to starts looking like the commercial
antenna I was initially studying. Their unit though only had a 1/4 wave
above the coaxial sleeve.

From the SMA plug, the coax coiled around the radome must have been the
choke to help reduce RF currents radiating from the earth braid.
Then the 1/4 wave length coax forms the match followed by the expose
radiating section.

It looks like an open stub fed J-pole where the stub is enclosing the
inner element rather than being constructed as a rod next to it.

Does this seem reasonable ? (PS. I'm a beginner with regards to antenna
theory and would like to understand what is happening so that I can
experiment with some kind degree of success).

Yes, it does. Take a look at the following:

http://download.antennex.com/hws/ws1002/sperrtof.pdf

A Sperrtof, in effect, is a J-pole whose matching section is a coaxial
tube rather than a single rod or wire. It sounds rather like what
you're describing.

As with all such (I think), the radiating section is 1/2 wavelength
long, give or take a smidge, and behaves as an end-fed 1/2-wave
dipole. The matching section isn't supposed to radiate significantly.

The overall radiation pattern would, I expect, be essentially the same
as other J-poles and other end-fed 1/2-wave radiators - similar to a
center-fed 1/2-wave dipole, but tilted a bit "upwards" away from the
feedpoint.

You can distinguish a Sperrtof-type antenna from one of the coaxial
dipoles Jerry referred you to, by the length of the single-wire
radiator - it's 1/2-wave for a Sperrtof and 1/4-wave for a coaxial
dipole (which is really a center-fed dipole).

There's an interesting dual-band 2m/440 antenna which was written up
in QST in October 2000 - ARRL members can get the article at
http://www.arrl.org/members-only/tis...df/0010050.pdf

It's interesting because it's _called_ a J-pole, _looks_ like a
J-pole... but electrically it isn't. It's actually a center-fed
vertical, not a Zepp. The stub at the bottom acts as a
choke/decoupler, not as an impedance transformer.

--
Dave Platt AE6EO
Hosting the Jade Warrior home page: http://www.radagast.org/jade-warrior
I do _not_ wish to receive unsolicited commercial email, and I will
boycott any company which has the gall to send me such ads!

Dave,

If I scale that up to 915MHz, the dimensions are almost perfect for
using RG58 coax instead of the copper tubing.The outer tube would be
5.5mm and inner conductor 1.6mm. Being 50 Ohm coax, I assume therefore I
do not need to slide the tap point up as their design did. I suppose I
should also add the choke coil at the antenna base for additional
isolation of ground currents.

The article mentions the top part as 1/2 wave and bottom at 1/4 wave. If
the bottom part of their design is 505mm then the antenna must be used
for 2m band. If this is the case, how come the radiating element is 1480
long ?

The Coaxial antenna mentioned in the article mentioned by Jerry looks
good. The version they mention with a GAP would be simple to construct.
It is a 1/4 wave radiating element at the top (earth braid stripped
back), then 1/4 wave of full coax, then a "small" gap, then the
transmission line. They do not mention the size of the gap but I assume
any small gap has the same effect of isolating the reverse current ?

Should the actual lengths of these sections be modified by "K" depending
on diameter of conductor or are they exact 1/4 wave length cuts ?

Thanks heaps.


Dave Platt wrote:
In article ,
David wrote:


The article Jerry pointed me to starts looking like the commercial
antenna I was initially studying. Their unit though only had a 1/4 wave
above the coaxial sleeve.

From the SMA plug, the coax coiled around the radome must have been the
choke to help reduce RF currents radiating from the earth braid.
Then the 1/4 wave length coax forms the match followed by the expose
radiating section.

It looks like an open stub fed J-pole where the stub is enclosing the
inner element rather than being constructed as a rod next to it.

Does this seem reasonable ? (PS. I'm a beginner with regards to antenna
theory and would like to understand what is happening so that I can
experiment with some kind degree of success).


Yes, it does. Take a look at the following:

http://download.antennex.com/hws/ws1002/sperrtof.pdf

A Sperrtof, in effect, is a J-pole whose matching section is a coaxial
tube rather than a single rod or wire. It sounds rather like what
you're describing.

As with all such (I think), the radiating section is 1/2 wavelength
long, give or take a smidge, and behaves as an end-fed 1/2-wave
dipole. The matching section isn't supposed to radiate significantly.

The overall radiation pattern would, I expect, be essentially the same
as other J-poles and other end-fed 1/2-wave radiators - similar to a
center-fed 1/2-wave dipole, but tilted a bit "upwards" away from the
feedpoint.

You can distinguish a Sperrtof-type antenna from one of the coaxial
dipoles Jerry referred you to, by the length of the single-wire
radiator - it's 1/2-wave for a Sperrtof and 1/4-wave for a coaxial
dipole (which is really a center-fed dipole).

There's an interesting dual-band 2m/440 antenna which was written up
in QST in October 2000 - ARRL members can get the article at
http://www.arrl.org/members-only/tis...df/0010050.pdf

It's interesting because it's _called_ a J-pole, _looks_ like a
J-pole... but electrically it isn't. It's actually a center-fed
vertical, not a Zepp. The stub at the bottom acts as a
choke/decoupler, not as an impedance transformer.

David wrote:
If I scale that up to 915MHz, the dimensions are almost perfect for
using RG58 coax ...

I wouldn't use RG58 for anything above HF. It has 20 dB
matched line loss per 100 ft at that frequency.
--
73, Cecil http://www.qsl.net/w5dxp

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So for the 1/4 wave (81mm) section of the antenna the loss would be
about 0.05dB ? at 915 MHz.

I typically have between 150mm (When antenna mounted on enclosure) and
2m length (when antenna external to enclosure) cable between transmitter
and Antenna. (Around 1.2dB max. loss in coax between transmitter and
antenna). Would this usually be acceptable or do you aim for much lower
loss in the transmission line ? Max. power is 1 Watt. Most of the
transceivers I play around with are around 10mW.

Thanks

Cecil Moore wrote:
David wrote:

If I scale that up to 915MHz, the dimensions are almost perfect for
using RG58 coax ...


I wouldn't use RG58 for anything above HF. It has 20 dB
matched line loss per 100 ft at that frequency.

"David" wrote in message
...
So for the 1/4 wave (81mm) section of the antenna the loss would be about
0.05dB ? at 915 MHz.

20dB/100' MATCHED line loss.
W4OP

David wrote:
So for the 1/4 wave (81mm) section of the antenna the loss would be
about 0.05dB ? at 915 MHz.

Depends upon whether you leave the insulation on the radiating
part or not. But what I was concerned about is the transmission
line. You lose about half of your power every 15 feet. Until
your last posting, you didn't tell us the length of the
transmission line but personally I find 1.2 dB matched line
loss in six feet to be unacceptable. Any SWR above 1:1 and the
losses are even greater.

Cecil Moore wrote:
I wouldn't use RG58 for anything above HF. It has 20 dB
matched line loss per 100 ft at that frequency.
--
73, Cecil http://www.qsl.net/w5dxp


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In article ,
David wrote:

Dave,

If I scale that up to 915MHz, the dimensions are almost perfect for
using RG58 coax instead of the copper tubing.The outer tube would be
5.5mm and inner conductor 1.6mm. Being 50 Ohm coax, I assume therefore I
do not need to slide the tap point up as their design did.

Bad assumption, I think. The portion of the matching section between
the tap point and the bottom is a shorted stub, which adds some
inductive reactance at the feedpoint. If you don't have it, you won't
get a good match.

Another approach I've seen to creating the necessary shorted stub is
to just use RG-58 and a simple BNC "T" connector. If you look at the
Sperrtopf antenna design, picture it made this way:

- one long piece of RG-58 1380 mm long, with the upper part of
the outer shield removed... that's your radiator, and the portion
of the matching section above the feedpoint.

- A second section of RG-58, about 100 mm long, with one end shorted.
That's the stub.

- The feedline RG-58.

Now, just connect all three together... radiator/matching section,
stub, and feedline. You can solder 'em, or add BNC plugs and use a
BNC "T". If you do the latter, remember to include the lengths of the
plugs and "T" into account.

I suppose I
should also add the choke coil at the antenna base for additional
isolation of ground currents.

Certainly would not hurt to do so. You could either coil the coax
below the bottom of the matching section, or add a quarter-wave choke
sleeve, or just run the coax through a few ferrite beads.

The article mentions the top part as 1/2 wave and bottom at 1/4 wave. If
the bottom part of their design is 505mm then the antenna must be used
for 2m band. If this is the case, how come the radiating element is 1480
long ?

I think there's some confusion in the antenna schematic diagram,
compared with the text and parts list. The diagram appears to show
the upper (radiating) portion of the center element being 1480 mm, but
the parts list indicates that the *total* length of the inner element
is 1480 mm.

The latter seems more correct to me... it'd give you a radiating
element of (1480 - 505) = 975 mm, which is a bit less than 1/2
physical wavelength at 2 meters. This seems reasonable to me given
that the radiating element is fairly thick (10mm) rather than a thin
wire.

The Coaxial antenna mentioned in the article mentioned by Jerry looks
good. The version they mention with a GAP would be simple to construct.
It is a 1/4 wave radiating element at the top (earth braid stripped
back), then 1/4 wave of full coax, then a "small" gap, then the
transmission line. They do not mention the size of the gap but I assume
any small gap has the same effect of isolating the reverse current ?

I'm not clear about the gap configuration either. The article sort of
implies that its size has to be set experimentally, so that the
leakage current through the gap is the right amount to cancel out the
signal propagating back down the outside of the upper section. The
article doesn't say whether the gap is supposed to go around the full
circumference of the coax (completely isolating the upper and lower
portions of the shield) or only partially around... I infer that it's
the latter, because I don't see how the antenna could work with the
shield sections fully separated.

Should the actual lengths of these sections be modified by "K" depending
on diameter of conductor or are they exact 1/4 wave length cuts ?

Well, I think it's a bit of a compromise.

The upper section (the coax center conductor and its surrounding
dielectric) is going to have a velocity factor of not much less than
1.0. Cutting to 1/4 physical wavelength, and then trimming a bit,
seems likely to work.

I think the lower section is likely to be a bit trickier, since
there are actually two velocity factors involved. The radiating is
done by the RF travelling back down the outside of the coax braid from
the feedpoint, and this (like the upper section) will have a velocity
factor close to 1.0, which suggests that the gap-to-feedpoint distance
should be close to 1/4 physical wavelength.

HOWEVER: the signal travelling up the *inside* of the braid, like that
travelling up the inner conductor towards the feedpoint, will be
travelling more slowly - the velocity factor will be somewhere around
..66 - .8 depending on cable type. This means that there will be more
than 180 degrees of phase delay between the signal passing the gap
going upwards (inside the cable) and the signal reaching the gap from
above (travelling back down the outside of the cable). Hence, the gap
leakage won't result in full cancellation of the signal travelling
down the outside of the coax past the gap.

To get the two signals into accurate 180-degree phase opposition,
you'd need to make the distance from the feedpoint to the gap a bit
less than 1/4 physical wavelength, but a bit more than 1/4 of the
coax's usual electrical wavelength. Splitting the difference might
work fairly well.

I'm not sure what this will do to the antenna's feedpoint impedance /
return loss, though, and it'll probably tilt the antenna's radiation
pattern somewhat. You will probably need to experiment to get the
best compromise between antenna pattern and feedpoint impedance /
return loss / SWR. The same is true of the gap size and configuration.

--
Dave Platt AE6EO
Hosting the Jade Warrior home page: http://www.radagast.org/jade-warrior
I do _not_ wish to receive unsolicited commercial email, and I will
boycott any company which has the gall to send me such ads!

On Thu, 15 Sep 2005 18:37:48 -0000, (Dave Platt)
wrote:

In article ,
David wrote:

Dave,

If I scale that up to 915MHz, the dimensions are almost perfect for
using RG58 coax instead of the copper tubing.The outer tube would be
5.5mm and inner conductor 1.6mm. Being 50 Ohm coax, I assume therefore I
do not need to slide the tap point up as their design did.

Using RG58 at 900mhz is a lossy situation to be avoided.

Bad assumption, I think. The portion of the matching section between
the tap point and the bottom is a shorted stub, which adds some
inductive reactance at the feedpoint. If you don't have it, you won't
get a good match.

True. You can alter the lengths and use the stub section to cancel
the inductive or capacitive reactance set up by the radiating element
if present. If your radiating element is capacitive (tad shorter)
then the stub can be open which will reflect as inductive. It's only
a matter of juggling the numbers.

Another approach I've seen to creating the necessary shorted stub is
to just use RG-58 and a simple BNC "T" connector. If you look at the
Sperrtopf antenna design, picture it made this way:

- one long piece of RG-58 1380 mm long, with the upper part of
the outer shield removed... that's your radiator, and the portion
of the matching section above the feedpoint.

- A second section of RG-58, about 100 mm long, with one end shorted.
That's the stub.

- The feedline RG-58.

Now, just connect all three together... radiator/matching section,
stub, and feedline. You can solder 'em, or add BNC plugs and use a
BNC "T". If you do the latter, remember to include the lengths of the
plugs and "T" into account.

I suppose I
should also add the choke coil at the antenna base for additional
isolation of ground currents.

Certainly would not hurt to do so. You could either coil the coax
below the bottom of the matching section, or add a quarter-wave choke
sleeve, or just run the coax through a few ferrite beads.

If the antenna feed point is a matched condition what is the RF on the
coax shield from? It should be very small.

The article mentions the top part as 1/2 wave and bottom at 1/4 wave. If
the bottom part of their design is 505mm then the antenna must be used
for 2m band. If this is the case, how come the radiating element is 1480
long ?

I think there's some confusion in the antenna schematic diagram,
compared with the text and parts list. The diagram appears to show
the upper (radiating) portion of the center element being 1480 mm, but
the parts list indicates that the *total* length of the inner element
is 1480 mm.

For 2m the correct length for the total center element is 1480 (58").
At 2m the halfwave length is in free space is 1M or for real elements
about 38" (.95M) or a K around .96-97.

The latter seems more correct to me... it'd give you a radiating
element of (1480 - 505) = 975 mm, which is a bit less than 1/2
physical wavelength at 2 meters. This seems reasonable to me given
that the radiating element is fairly thick (10mm) rather than a thin
wire.

Correct. Thick elements do make a noticeable difference especially
when the ratio of wavelength to element diameter gets below 500:1.
The effect is elements are shorter, bandwidth wider.

The Coaxial antenna mentioned in the article mentioned by Jerry looks
good. The version they mention with a GAP would be simple to construct.
It is a 1/4 wave radiating element at the top (earth braid stripped
back), then 1/4 wave of full coax, then a "small" gap, then the
transmission line. They do not mention the size of the gap but I assume
any small gap has the same effect of isolating the reverse current ?

There is no requirement for the bottom to be coaxial. it can be
parallel lines (convential Jpole) in either case the antenna is a
1/2 lamda using a open section of transmission line for matching.

I'm not clear about the gap configuration either. The article sort of
implies that its size has to be set experimentally, so that the
leakage current through the gap is the right amount to cancel out the
signal propagating back down the outside of the upper section. The
article doesn't say whether the gap is supposed to go around the full
circumference of the coax (completely isolating the upper and lower
portions of the shield) or only partially around... I infer that it's
the latter, because I don't see how the antenna could work with the
shield sections fully separated.

The gap is fully open. Also that gap sets up a transmisstion line
between the outer braid and the inner transmission line braid.
There is an interaction between the gap and braid section length.

Should the actual lengths of these sections be modified by "K" depending
on diameter of conductor or are they exact 1/4 wave length cuts ?

Yes. however the difference between K and free space will be maybe
a 2-4% difference. At 2m that comes out to around .5" (12mm) give or
take.

Well, I think it's a bit of a compromise.

The upper section (the coax center conductor and its surrounding
dielectric) is going to have a velocity factor of not much less than
1.0. Cutting to 1/4 physical wavelength, and then trimming a bit,
seems likely to work.

Even without insulation it's lower than 1.0 (likely .98 or so)
but insulation will increase that.

I think the lower section is likely to be a bit trickier, since
there are actually two velocity factors involved. The radiating is
done by the RF travelling back down the outside of the coax braid from
the feedpoint, and this (like the upper section) will have a velocity
factor close to 1.0, which suggests that the gap-to-feedpoint distance
should be close to 1/4 physical wavelength.

HOWEVER: the signal travelling up the *inside* of the braid, like that
travelling up the inner conductor towards the feedpoint, will be
travelling more slowly - the velocity factor will be somewhere around
.66 - .8 depending on cable type. This means that there will be more
than 180 degrees of phase delay between the signal passing the gap
going upwards (inside the cable) and the signal reaching the gap from
above (travelling back down the outside of the cable). Hence, the gap
leakage won't result in full cancellation of the signal travelling
down the outside of the coax past the gap.

To get the two signals into accurate 180-degree phase opposition,
you'd need to make the distance from the feedpoint to the gap a bit
less than 1/4 physical wavelength, but a bit more than 1/4 of the
coax's usual electrical wavelength. Splitting the difference might
work fairly well.

I'm not sure what this will do to the antenna's feedpoint impedance /
return loss, though, and it'll probably tilt the antenna's radiation
pattern somewhat. You will probably need to experiment to get the
best compromise between antenna pattern and feedpoint impedance /
return loss / SWR. The same is true of the gap size and configuration.

If properly matched the feedpoint will be 50ohms and the return loss
will be reflective of a good match.


Allison
Kb1GMX

On Thu, 15 Sep 2005 19:13:28 GMT, wrote:

If the antenna feed point is a matched condition what is the RF on the
coax shield from? It should be very small.

Hi OM,

You have just hit the trip-wire of a common misunderstanding.

73's
Richard Clark, KB7QHC