On Nov 7, 4:18*pm, Owen Duffy wrote:
"David" nospam@nospam wrote :



Which antenna is better: 5/8 wavelength vertical or a J pole?

Better for what? For example, a 5/8 whip would usually be better than a J
Pole for mobile applications on a car roof, but that doesn't make it
better for all applications.



Frequency of operation is 145 MHz *= 2 metres.

The 5/8 wavelength vertical has a loading coil. There are losses in
the coil.

It is certainly popular to talk down an antenna with loading coils or
traps because they are "lossy".

Everthing in a real world antenna has loss, the issue is the magnitude of
the loss, and the impact of that on system performance for the intended
application.

You might find it hard to believe that some antenna systems incorporate
loss elements in order to reduce feed line loss by more than that in the
introduced elements.

Whilst you have chosen to raise the loss in the coil, you haven't raised
the issue that a J Pole has currents flowing in lossy conductors,
components of which that do not directly contribute to radiation.



The J pole has a quarterwave matching stub. The matching stub provides
an out of phase current which means that there is a cancelling field
close to the radiating element. Also the J pole is end fed, which

This is not a very good way of analysing the J Pole. The U section can be
thought of as carrying currents that have differential and common mode
components. The common mode components contribute directly to radiation
field. You should also consider common mode current on the supporting
structure and feedline.

The 5/8 wave vertical also has potential for significant common mode
current on the supporting structure and feedline, you need to look at the
effectiveness of the decoupling method employed (usually a radial set).

means the transmitter is not connected directly to a maximum current
point.

What does that matter? BTW, neither is the base fed 5/8.

There is a maxim in ham radio that antennas should always be fed at a
current maximum. You could subscribe to that, but you would limit
yourself by excluding a range of good solutions, and with no guarantee
that a current fed antenna is optimum.



Does the J pole have a disadvantage because of *the cancelling field

There is not perfect cancellation at all points on the U section.

from the matching stub and the fact that it is end fed?

"End fed", as opposed to a centre fed dipole perhaps. But isn't the 5/8
"end fed"?



Also consider gain and angle of radiation.

The three dimensional gain distribution is important, but dependent on
the common mode issue mentioned above (amongst other things).

It is fair to say that J Poles are very popular, and that they are overly
represented in problems discussed in online fora.

On the other hand, the 5/8 which was once very popular for mobile work in
this area, is long lost, replaced by two, three and four band antennas
where VSWR is more important than any other performance parameters. I use
a 5/8 vertical on my car, and regularly work repeaters mobile at
distances well over 100km. The antenna is more than thirty years old, and
has never required repair despite hitting low trees, carpark roofs etc
lots of times. The modern multiband antennas are not that robust. I
wouldn't even think of a J Pole in this application.

So, "best" can be a quite complex requirement.

Owen

Owen, I think the popularity of VHF mobile 5/8 antenna lies in it has
more gain than a 1/4 wl antenna and is easier to match to 50 ohms than
a .5 wl antenna. I had certainly rather DIY a 2M 5/8 mobile antenna
than a .5 wl version. From information I have seen the 5/8 often
touted for its low angle of radiation may actually have a
significantly higher angle of radiation than the .5 wl antenna used in
a similar situation. This is not to say either antenna would not be
equally useful.

Jimmie