Does anyone have any experience using the MFJ-1795 vertical antenna?
Because of it's size, it's probably one of the few antennas that I will be
able to put in my back yard.

I'd like to hear any comments on using this antenna, performance, etc...

Thanks!

"notme" wrote in
:

Does anyone have any experience using the MFJ-1795 vertical antenna?
Because of it's size, it's probably one of the few antennas that I
will be able to put in my back yard.

I'd like to hear any comments on using this antenna, performance,
etc...

Firstly, I don't have first hand experience with the MFJ1795, however
some thoughts...

It appears from their description and manual to be a vertical of 7 to 10
feet in height, and with top loading. They describe options for mounting
on the ground, or an elevated mount with radials.

The challenge with any short ground mounted vertical is to increase the
radiation resistance of the vertical and decrease the equivalent loss
resistance of the ground connection.

Top loading helps increase radiation resistance, but the radiation
resistance of a 10' radiator on 7MHz will be low relative to practical
ground system resistances, and typical system efficiency with a minimal
ground system is unlikely to be greater than about 25%. However, that
might be a quite acceptable compromise to many users.

MFJ claims: 'The MFJ-1795 has a low angle of radiation for DXing and is
omni-directional. The MFJ-1795 handles 1500 Watts PEP. It has highly
efficient end-loading. The entire length of the antenna radiates and it
has low SWR.'

Sounds impressive, but a low angle of radiation will require a superb
ground system, it is likely to be omni-directional if there are no nearby
obstructions, radials will distort the pattern if there a few in number,
end loading might have some loss advantages, but ground resistance may
swamp loading loss and largely determine the antenna efficiency.
Improving the ground system may increase VSWR and increased line loss
might eat away some of the gains.

Vertical antennas are often quite noisy relative to a horizontal antennas
in built up areas, something to consider.

Owen

Owen Duffy wrote in
:

Top loading helps increase radiation resistance, but the radiation
resistance of a 10' radiator on 7MHz will be low relative to practical
ground system resistances, and typical system efficiency with a
minimal ground system is unlikely to be greater than about 25%.
However, that might be a quite acceptable compromise to many users.

Deep in the manual, I find:
1. the expectation that VSWR on 40m is likely to be around 1.2, and
2. that the unloaded part of the radiator is adjustable from 5' to 8'.

Following from 2, modelling indicates that a top loaded 8' vertical
probably has a radiation resistance around 5 ohms on 7MHz.

From 1, assuming that VSWR 2 is at the feed point and is achieved with a
feed point resistance of 42 ohms, then efficiency at the feedpoint = 5/42
or 12%.

The VSWR is so good that line loss with 100' of RG8/X for example should
give an antenna system efficiency of around 10%.

Owen

On Thu, 29 Mar 2007 21:12:22 -0500, "notme" wrote:

Does anyone have any experience using the MFJ-1795 vertical antenna?
Because of it's size, it's probably one of the few antennas that I will be
able to put in my back yard.

I'd like to hear any comments on using this antenna, performance, etc...

Thanks!



I don't have the antenna, but I downloaded the manual for it. Is this
a multi-band antenna or one band at the time?

I see on page 7 what looks like a shorting strap across all coils
except the 40 meter coil...

thank you,

--
73 for now
Buck, N4PGW
www.lumpuckeroo.com