On 31 ago, 08:56, "neno" wrote:
Hello,
I build lambda/4 vertical for 20m (i used the fishing pole).
The vertical and radials are build from insulated wire 2.5 square
millimeters.
I have used formula on internet and Eznec so I cut 5,09meters wire for
vertical and 4 pieces 5.25m wire for radials (i left radials longer so I can
cut them to make fine tuning of antenna).
I wanted minimal SWR on 14.050MHz.
But now I have minimal SWR on 12.800 MHz . :-)
According to Eznec if I short the radials for 2meters (so they will be long
3.25m) that will rise resonant frequency around 1.2MHz.
But it is not logic to me. So short radials?
Also according to Eznec if I short vertical to 4.7meters and radials to
4.7meters that will also rise resonant frequency arround 1.2MHz.
Could that be that my mistake is that I didn't calculate the velocity factor
for wire.
Maybe i should calculate it because I used single wire instead of aluminum
tube.
Or maybe some surrounding objects are influencing. I have metal mast for
loop antenna 5meters from vertical.
It is easier to cut radials (because I don't have to take down the whole
antenna).
What do You suggest?
To cut only radials to 3.25m or vertical and radials to 4.7m?
What is better?
Just to mentioned that I tuned antenna with tuner and it works great for
DXing.
73 9A4MN Neno

Hello Neno,

Influence of PVC wire insulation on wave propagation is just a few
percent. Most of the field lines go through air. I did a calculation
myself (however results are in Dutch language). When the wire is
thicker, the influence of same layer of insulation becomes less.

The soil has significant influence on resonant frequency and steepness
of resonance of the radials. When you use many of them the length
becomes less critical. You can also use elevated radials (for example
about 4 radials 1..2 m above ground).

However when VSWR is OK with short radiator or shorter radials and you
don't have common mode problems in the coaxial feed line, I would not
worry.

When you are still curious, you might model the nearby structures in
your NEC file and try to simulate the influence. A mast at 5m distance
is within the near field, so you may expect some coupling.

Instead of cutting, you can also roll them up (beginning at the end),
so you can re-extend them when required).

Best regards,

Wim
PA3DJS
www.tetech.nl