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W6GQ
05-19-2007, 01:11 PM
I bought a multi tap MFJ 1899T antenna for my 817 at Dayton and I am cutting some counterpoise wires.

I have always used quarter wave for my vertical counterpoise wires.

234 / freq = length in feet

For example:

10 meter = 234 / 28.500 = 8.2 ft

The MFJ manual states
Quote[/b] ]The best length for the ground plane wire is less than 1/4 wave. 180 freq (in MHz) = length in feet. The telescopic can be adjusted for best VSWR.

So with that information,

180 / 28.500 = 6.3 ft

Do you think this information specific to the MFJ -1899T because of the antenna design or should I have always used 180 / freq ?

What am I missing?

W6GQ
05-19-2007, 01:18 PM
Wait a minute,


Quote[/b] ]AFTER FURTHER REVIEW,,,,,,,,,,

Lengths of telescopics are for lowest frequency in band and with ground plane
wire connected to radio.

So they must want the buyer to use 180 because they say to connect the counterpoise to the radio ground lug in the back?

I do not feel comfortable connect it to the radio,,,,,should I not feel comfortable doing that?

I was just going to set up the antenna on a small camera tri-pod bought at a garage sale for $1 http://www.qrz.com/iB_html/non-cgi/emoticons/biggrin.gif ?

AI4IJ
05-19-2007, 03:46 PM
Quote[/b] (W6GQ @ May 19 2007,06:18)]Wait a minute,


Quote[/b] ]AFTER FURTHER REVIEW,,,,,,,,,,

Lengths of telescopics are for lowest frequency in band and with ground plane
wire connected to radio.

So they must want the buyer to use 180 because they say to connect the counterpoise to the radio ground lug in the back?

I do not feel comfortable connect it to the radio,,,,,should I not feel comfortable doing that?

I was just going to set up the antenna on a small camera tri-pod bought at a garage sale for $1 #http://www.qrz.com/iB_html/non-cgi/emoticons/biggrin.gif ?
If the antenna connects directly to the radio, then you will need a counterpoise connected directly to the radio. #And, it might very well need to be calculated based on 180/F, when taking the radio case into consideration. #But, if the antenna is remotely mounted to a tripod, the counterpoise radials should be calculated at 234/F and then shortened for best SWR - as installed.

73
Richard
AI4IJ

K7FE
05-19-2007, 04:09 PM
It seems to me that they are incorporating the outer portion of the coax shield into the equation and it becomes part of the radial/counterpoise system . That is why the "radial" is connected to the transceiver case. I noticed that they also recommend driving a stake into the ground for improved results. Think of it as a mobile antenna and not a home station dipole when you consider performance.

The manual is minimal.
www.mfjenterprises.com/man/pdf/MFJ-1899T.pdf

Just enjoy your purchase and operate from many interesting locations.

Radials on "full size" (1/4 or 5/8) vertical antennas should be 1/4 wave. Longer radials like 1/2 wave may have a small efficiency improvement. It is unlikely that on the air amateur radio signal reports will show any improvement, and thus is seldom done.

Short radials (less than 1/4 wave) may be used with reduced performance. MFJ suggested radial lengths are for a unique MFJ antenna only, so stick to 1/4 radials for your other vertical antennas.

73,
Terry, K7FE

WC5CW
05-19-2007, 10:36 PM
W6GQ, et al...

Re: Counterpoise for portable base loaded vertical radiators

>"I was just going to set up the antenna on a small camera tri-pod bought at a garage sale for $1"<-- W6GQ

I've used a homebrew 1/4-wave vertical mounted atop telescopic photography lightstand on several occasions while operating 2m FM during public service events and it has worked in an excellent manner...The radiator and four 1/4-wave radials were made from 16 awg piano wire having a ring #connector crimped onto the tips for personal protection...It's easily moved about and quickly assembled and disassembled using machine screws and wing nuts...When disassembled it fits neatly inside a cardboard tube that also carries the lightstand and connecting coaxial cable. #

Fundamentally, it has been my impression and casual observation based on personal experience that a counterpoise of at least 3 and ideally 4 or more radials of 1/4-wavelength (equally spaced as about the points on a compass) at the lowest operating frequency is the means to an effective vertical radiator...And furthermore, that the radials do not necessarily need to be physically connected to the radio transmitter...Indeed, a wire grid or mesh of suitable density having a 1/2-wavelength diameter (calculated) at the lowest operating frequency with the radiator placed at the center is pretty much the ideal counterpoise system...Adjustment of actual feedpoint Z and hence the VSWR can be accomplished by varying the height of the vertical radiator above the vertex of the radial system or the center of the wire mesh for a given operating frequency.

Others may critique or otherwise comment on this impression.

FWIW

Bruce
WC5CW

W6GQ
05-20-2007, 05:13 AM
Quote[/b] (wc5cw @ May 19 2007,15:36)]W6GQ, et al...

Re: Counterpoise for portable base loaded vertical radiators

>"I was just going to set up the antenna on a small camera tri-pod bought at a garage sale for $1"<-- W6GQ

I've used a homebrew 1/4-wave vertical mounted atop telescopic photography lightstand on several occasions while operating 2m FM during public service events and it has worked in an excellent manner...The radiator and four 1/4-wave radials were made from 16 awg piano wire having a ring #connector crimped onto the tips for personal protection...It's easily moved about and quickly assembled and disassembled using machine screws and wing nuts...When disassembled it fits neatly inside a cardboard tube that also carries the lightstand and connecting coaxial cable. #

Fundamentally, it has been my impression and casual observation based on personal experience that a counterpoise of at least 3 and ideally 4 or more radials of 1/4-wavelength (equally spaced as about the points on a compass) at the lowest operating frequency is the means to an effective vertical radiator...And furthermore, that the radials do not necessarily need to be physically connected to the radio transmitter...Indeed, a wire grid or mesh of suitable density having a 1/2-wavelength diameter (calculated) at the lowest operating frequency with the radiator placed at the center is pretty much the ideal counterpoise system...Adjustment of actual feedpoint Z and hence the VSWR can be accomplished by varying the height of the vertical radiator above the vertex of the radial system or the center of the wire mesh for a given operating frequency.

Others may critique or otherwise comment on this impression.

FWIW

Bruce
WC5CW
Thanks for the info, I will be sticking with the wire only because I want to keep the portability of the FT-817 and of the antenna, great info though.

The camera tri pod will hold the HF antenna.

WD8OQX
05-20-2007, 11:23 PM
I wouldn't trust Made From Junk as far as I can throw them!!!!!! - everything of there's that I've ever come in contact with has been a poorly designed POS!!!


GO WITH YOUR INSTINCTS ON THIS ONE!!!

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