The idea is that when the required frequency is applied, current flow through the dipole cut for that particular band, as the other dipoles present a high impedance, like us all why do it the hard way when the easiest is well, easier.
The original design was for 40/20 and 10m, fed with twin feeder, as I only wanted 40 and 20m and to feed it with coax, I have installed a 1:1 current balun, and cut wires to around 20.5 metres for 40m band and 10.3m for the 20m band.
To keep things simple I purchased the balun and dipole centre, and used some multi strand wire I had around as you do, for the spacers I used 1" diameter pvc pipe cut into sections and 3 holes drilled in each, one for the rope, and the other 2 for the 40/20m wires.
The ends were allowed to hang down around 10cm, this allowed for trimming for a low SWR mid band, the dipole has the following swr readings according to my MFJ analyzer..
7000 - 2.5
7200 - 1.4
7500 - 2.5
I can see I need to lengthen the 7Mhz elements, so it shows around 500 Khz bandwidth for a 2.5:1 swr, easily good enough for the 40m band of 200 Khz.
With regards to 20m I got the following readings -
14000 - 2.5
14200 - 1.2
14400 - 2.5
So again very useable from 14000 to 14350, as I dont do CW I wasnt too worried about the swr at the bottom of the band or for data modes as this antenna will be used for portable and garden op's !
According to the research I have done, the diploe should also be resonant on 21 Mhz, as my analyzer only tells the swr and not the impedance etc, I cant be sure that it is or isnt, however the swr is around 4:1 on 15m, with a low swr at 22 Mhz.
No contacts as yet when tried yesterday, and much warmer indoors in the shack today !
Hi Paul, looks good, now we need more propagations. As long the solar flux is below 80...
ReplyDelete73 Paul
Tell me about it Paul, my bits of wire are struggling at the moment.
ReplyDeleteHello Paul,
ReplyDeleteWhat a great-looking antenna. You should be very proud. Shame about 15m but perhaps that can be tweaked a bit...... anyway 20 and 40 are the main worries. Great pictures of the antenna too.
Thanks for your help trying to call me on Tuesday. Pity we couldn't make it but glad we tried. Next time I will go upto one of Dad's higher fields and try and open the Liskeard repeater.
Bye for now, Adam
PS
repeater list very useful especially having it laminated too. Thanks for that.
Hi Adam,
ReplyDeleteAs the 40m element needs lenghtening, then I am hoping it will also bring down the resonant frequency for the 15m band also.
Yes a shame re the sked, on 80m there was quite a few 'g' stations about, not sure about the extra 80m coil for the MP1, my experience of coils on the lower bands is one of extremely poor bandwidth, and I suspect efficiency.
No problem with the sheet, I wish now I had made it slightly smaller, no quite so handy as it says 'on the tin'.
Yes Adam, as 2E0XVI said to me recently on a totally different matter - head for the hills.
73
Paul.