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@gmail.com> wrote:
>
> 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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