The producer of "sommerantennas" is a SK. So informations are poor.
I am using a XP707. That means a 7 band beam from 40 mtrs up to 10 mtrs - all 7 bands. But of course... the 40 and 30 mtr kid do not work so good as the rest of upper bands.
What fouls you can have?
18 Mhz is an sensitive band. Therefore look here.
- if your SWR is wobbly in windy wx, you lost the plastic connection between the 15/17/20 mtrs elements
- if your SWR is dropping away with your power, change this plasic tube connection to a simple 5mm glasfiber connection
- if your SWR is jumping away, look for crackers mostly between feedline and ground
What extension you can do ?
1. 6 mtr extension
That's so easy you won't believe it. Take a 2,95 mtrs long aluminium tube. Put this tube under the shortest
element at the top of the beam below the boom. Yes, thats all.
It will work good enough. But you can do more, if you like...
What about that... still two directors more. No influence to other bands.
After 3 years working time I bought a separate 5 el yagi (8,7 dBd) for 6 mtrs and installed it 1,5 mtrs above my good old antenna.
I was astonished very much and won't believe it. My upper described antenna was 1 s-stage better in reports in and out as the new one...
2.a Easy 30 mtrs variation
The problem is not so much the shorted dipol. It's more the stub because you can only run low power with that.
Remove the origin 30 mtrs stub (not the short circuit frames).
Make the last and longest element (20 mtrs reflector) still longer.
For testing I used 2 glass fibre fishing rods each of 2,1 m, overcovering the tops of elements each about 40 cm.
A copper wire is fixed at the rods.
Present I use right tubes. The origin element is 5,80m, the extension 1,7m on each side.
So you have a full size element with directional affect. .. and you can work full power.
On 30: Gain 7,6dBi, front to back 2,8 dBi. SWR 1:1,3 like all other bands too.
Alteration on 20m: 0.6dBi less f/b (EZNEC) than before.
2.b 30 mtrs extension
If you like to save the 0,6dBi and put some more on:
Remove the 30 m stub (not short circuit frames).
Make last element longer like above in 2.a but without the copper wire (only glass fibre rods).
Make the boom longer appr. 1,8 till 2,5 m. Herefore I took the thicker end of my fishing rods.
Now you can stretch a 14.85 m long wire from one end of the glass fibre rod over your elongeted boom top to
the other end of your elongeted element. The fibre rods will be bended.
But before you do that you should prepare the feed line first. Also the feed line must be elongeted.
The feed wires must be crossed. The feed point is appr. 1,90 mtrs from
the middle of half wave wire. All wires should be light.
