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grahamhall123(at)gmail.co
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 Posted: Sun Jan 19, 2014 9:21 am Post subject: Dipole aerial |
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Can anyone help please.
I am trying to tune my dipole aerial as per instructions from Europa to my krt-2 radio using a swm.
My initial reading were
Swm 1.7 on 118.00 frequency
5 on 127.00
2.6 on 136.9
I therefore started trimming 3 mm at a time off the ends of each copper tape.
This caused the lowest frequency swm reading to increase, the middle range to fall and the 136.9 also to fall.
The results for each 3mm trim were
118.00. 127.00. 136.9
1.4 5. 2.4
1.6. 4.2. 2.0
2. 4.0. 1.9
2.4. 3.0. 1.6
3.0. 2.6. 1.4
At this stage I lost my nerve and stopped trimming the aerial!
Europa instructions say middle range frequency should be lowest swm reading.
Can any of you out there advise on what I've done or indeed should do?
Best wishes to you all.
Graham.
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deford.dave(at)gmail.com
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| Posted: Sun Jan 19, 2014 10:43 am Post subject: Dipole aerial |
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Graham,
You are right to be dubious about your results, as they don't reflect the
behavior of an undisturbed dipole antenna. Readings like these are most
often caused by current flowing on the outside of the shield of the coax
feed line. This is why we put toroid cores around the coax near the feed
point of the antenna. If the size and material of the core are
satisfactory, the outside current is blocked. What you read in this case
reflects the characteristics of the antenna by itself, rather than the
antenna plus the feed line, which acts like another antenna connected in
parallel with the dipole.
The easiest way to determine whether your toroid core is effective is to
simply grab the coax in your hand near the end that connects to the radio
(where the swr meter should also be located), while watching the meter
indication. Your should be able to run your hand along the coax over a
length of about a meter with minimal disturbance in the swr meter reading.
Alternatively, changing the length of the feed line by about half a meter
should have little effect on the reading. If this is not the case, you need
a more effective toroid. Ferrite cores are generally better than powdered
iron, and larger cores are more effective than smaller cores, assuming the
same inner diameter and core material. You should also try not to disturb
the antenna with metal objects and human bodies within 3 meters or so.
Good luck!
Dave DeFord
N135TD
[quote] --
| | - The Matronics Europa-List Email Forum - | | | Use the List Feature Navigator to browse the many List utilities available such as the Email Subscriptions page, Archive Search & Download, 7-Day Browse, Chat, FAQ, Photoshare, and much more:
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grahamhall123(at)gmail.co
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| Posted: Mon Jan 20, 2014 1:31 am Post subject: Dipole aerial |
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Hi Dave,
Thanks for most helpful reply.
When time and weather allow I will give a go!
Will let you know how it goes.
Regards
Graham.
On Jan 19, 2014, at 6:43 PM, "David DeFord" <deford.dave(at)gmail.com> wrote:
[quote]
Graham,
You are right to be dubious about your results, as they don't reflect the
behavior of an undisturbed dipole antenna. Readings like these are most
often caused by current flowing on the outside of the shield of the coax
feed line. This is why we put toroid cores around the coax near the feed
point of the antenna. If the size and material of the core are
satisfactory, the outside current is blocked. What you read in this case
reflects the characteristics of the antenna by itself, rather than the
antenna plus the feed line, which acts like another antenna connected in
parallel with the dipole.
The easiest way to determine whether your toroid core is effective is to
simply grab the coax in your hand near the end that connects to the radio
(where the swr meter should also be located), while watching the meter
indication. Your should be able to run your hand along the coax over a
length of about a meter with minimal disturbance in the swr meter reading.
Alternatively, changing the length of the feed line by about half a meter
should have little effect on the reading. If this is not the case, you need
a more effective toroid. Ferrite cores are generally better than powdered
iron, and larger cores are more effective than smaller cores, assuming the
same inner diameter and core material. You should also try not to disturb
the antenna with metal objects and human bodies within 3 meters or so.
Good luck!
Dave DeFord
N135TD
> --
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