Topband: Butternut HF-2V - Freq. Drifting Problems (Javier - YV5MBX)

[Plasticos Omega]

Hi,

Just a brief note to share with you some inconveniences that I encountered since I installed the 160mt Kit into my Butternut HF-2V w/240 radials that instead those inconveniences had worked as good as my Inverted-L.

Below the Technical Issue sended to Bencher (This issue was answered extremly fast from THEM!!!)

MESSAGE  ************************************************
At the time of TBR-160 installation, I the minimun swr achieved was 2.3 at 1.830 before this I try without the base small coil that goes between the ground and the antenna by itself and the swr were high.

Then I thought that the key factor for a lower swr achievement were at the base coil so I changed with a higher value coil made at  home using cooper cable (# 12) over a PVC tube the as result I achieved 1.3 swr at 1.830

At the last 160 contest (last week-end)  after 1 hour operation I observed a change in the swr pattern, so the swr started to rise at 1.830 from 1.3 to 2.1 and then at 1.840 the swr were 1.1 and this value remains for 7 hours of operation . That happened on Friday evening.

The same happened on Sturday.

At first glance I thought that something had happened with my swr meter so I changed with a diferent one and the results were the same.

The questions are:
Why could be happen this?????
There is no more energy waste as the base coil is bigger???
Do you have any recommendation for a real good base coil?????

END************************************

REPLY**********************************
Your problems with the drift in center frequency have nothing to do with 
the base coil you made. Instead, they relate to the 200 pf capacitors 
used in the assembly.

These capacitors have an N-750 temperature coefficient, which means that 
their value shifts as they warm up. Since the value shifts the center 
frequency shifts with it. Since you were doing a lot of transmitting 
because of the contest, your capacitors warmed up due to the currents 
going through them and the frequency changed.

There is no simple fix for this. Unfortunately, NPO - zero temperature 
coefficient - capacitors are not available in this value and power 
rating. The 67 pf capacitors used in some of our other antennas do have 
NPO characteristics and 3 of them could be used in place of each 200 pf 
capacitor, but this would be quite expensive.

In any case, that is the source of the problem.
END OF MESSAGE*********************************

Hope this information will be useful for some of you!!!!!

73's
Javier
YV5MBX