Homemade Antenna Help

Glad it helped.

The short simple answer is yes you can. The harder questions are should you bother and if you bother how much do you get performance wise?

I"m still working on those myself.
In general if you aren't planning on pushing a lot more power than normal CB 4-12 watts you should probably just be happy with a very acceptable SWR.
BTW 1.4 SWR doesn't cost you as much as it may seem.

The gain improvement over the shorter firestick is probably more than the loss caused by the SWR mismatch.

Ways to get a "better" SWR .
Make the bottom part of the dipole you've made ( the jeep ) look electrically more like the ground plane antenna with the 4 radials each ~104" drooping at ~ 45 degrees. Like the ground planes WA4GCH posted pics of. But I doubt you want to try that.

Antenna tuners show a "perfect" match to the radio so it wont reduce power or be damaged due to high SWR.

Matching networks also match the antenna to the transmission line/coax.

But unless you just want to play they are probably not worth the effort.

I doubt you will be able to see the difference in performance achieved by lowering the SWR any more.
But that is just opinion.

 

The velocity through steel is much different than through copper, thereby rendering your original measurements useless. If you don't have an antenna analyzer, you will have to do this by trial and error. With the price of copper, I would eliminate all the guesswork by borrowing or buying an MFJ-259. It will tell you where you are resonant, and based upon that reading you will know whether to make it longer or shorter. I believe the velocity factor through steel is greater than the velocity through copper. If so, your antenna is too long.

 

I'm not sure velocity factor is the right term. because the dielectric in this case is air for both metals (no insulation) although the fiberglass reinforcement rods could be causing a small change in velocity of propagation.

But yes I take your point regarding metal conductivity. Copper is more conductive than steel (or any metal except silver).

if you make two dipoles both 12 gauge wire the one made of stainless steel would get best SWR ~1.49 at 104.5 "

The one made of copper would be optimum at ~1.44 and 104.67" long.

That's less than a quarter inch difference.
If you could make one from a "perfect" conductor the optimum length would be 104.75 " (SWR is still 1.44)

Considering this is a mobile antenna the shape of his ground plane is probably more significant.
Velocity factor is the ratio of length of a wavelength in a conductor/coax divided by the length of a wavelength in free space. Its always less than one, and the higher the VF the longer the conductor needs to be.

For un-coated/uninsulated wires a VF of 1 is close enough.

 

3.1 SWR.....Think I would check my coax and connectors for ground short first...could be that there's nothing wrong with your home-made antenna contraption...

 

The OP, Fudz073, already stated in post # 10 that he managed to reduced the SWR to 1.4 by cutting the element down to 104".
And was asking about reducing it farther.

 

I know 102" is the lenth for 11 meters

 

102" because stainless steel whips are commonly sold that long?

In air or vacuum at 11 meters the length of a quarter of a wave is about 108"

11 meters x (100 cm/meter) x (1 inch/ 2.54 cm) = 1100/2.54= 433.07 dividing that by 4 for a quarter wavelength gives 108.27, but that's in air.
In metal the signal is slower ( or something like that) so I misspoke, a fudge factor should be applied to the wavelength equation for metal vs air.

Notice that 104"/108.27= .96
if you go to this online wavelength calculator and pick "insulated wire or end effects dipole" it will use a "velocity factor" of .96

I think most antenna calculators include this factor (even if its hidden in the equation) I got the ~104 from optimizing a dipole model.
The modeling program (4nec2) has a drop down menu that lets you select what metal your model is made from.

And run sweeps like this one made of copper, the numbers along the bottom are in inches

This one uses perfect conductors

 

The spring makes the 102"s 108 on the SS. Fiberglass whips was 96".

 

my bad......wife keeps telling me I need to have my eyes checked...

 

You should borrow or purchase an MFJ 259-B in order to see the resonant frequency of the element to determine if it is long or short in relation to the desired frequency. Different metals require different lengths to be resonant at the same frequency.
unless your calculations were close, an swr meter will be of little use in designing an antenna. You really need an antenna analyzer to see the big picture.