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
> -----Original Message-----
>
> --> <grahamhall123@gmail.com>
>
> 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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