Need some input..

[patmcc24]

I am attempting to build an antenna for a scanner setup similar to this https://www.qsl.net/n4yek/scanner/antenna.pdf
However I have changed the dimensions mainly in the hopes to improve the gain and to use up some old things I have just laying around.
The center is 150Mhz - 49.1" 5/8 wave. UHF is 450Mhz - 16.4" 5/8 wave. And finally 800Mhz -9.22" mainly I guess I am asking am I on the right Track? and where exactly should the 800Mhz go? Sorry it has been a very long time and my math and memory is not what it used to be. Also reason for use of said items is I have an NMO mast mount and the NMO base with 30" radials at my disposal.... The coil base has been gutted and modified for thru feed.

 

[Ubbe]

You'll need a loading coil at the base as you cannot connect a 5/8 antenna directly to the base. Look at same 5/8 GP antennas schematics and self builders experiments to get proper windings and tap point.

/Ubbe

 

[patmcc24]

even as receive only it must be loaded?

 

[Ubbe]

The principals of how an antenna works applies independent of what you connect its coax to.

/Ubbe

 

[N8IAA]

I am attempting to build an antenna for a scanner setup similar to this https://www.qsl.net/n4yek/scanner/antenna.pdf
However I have changed the dimensions mainly in the hopes to improve the gain and to use up some old things I have just laying around.
The center is 150Mhz - 49.1" 5/8 wave. UHF is 450Mhz - 16.4" 5/8 wave. And finally 800Mhz -9.22" mainly I guess I am asking am I on the right Track? and where exactly should the 800Mhz go? Sorry it has been a very long time and my math and memory is not what it used to be. Also reason for use of said items is I have an NMO mast mount and the NMO base with 30" radials at my disposal.... The coil base has been gutted and modified for thru feed.View attachment 68463

If you build the antenna using what you have, that coil, which is for the VHF element, won't work. There is no way to tie in the other elements that will allow the coil to pass the signal.
If you build it according to the link you provided, that antenna will work. It's just simple antenna physics.

Larry

 

[N8IAA]

You'll need a loading coil at the base as you cannot connect a 5/8 antenna directly to the base. Look at same 5/8 GP antennas schematics and self builders experiments to get proper windings and tap point.

/Ubbe

He does have a coil. Unfortunately, it is for the VHF 5/8 antenna in the picture.
Larry

 

[mancow]

If you look at a scantenna is basically the same design but in a dipole configuration. Not relevant but thought it was interesting.

 

[prcguy]

The OP mentioned he gutted the coil and its straight through now. For the original 5/8 wave VHF antenna the coil is needed. For the stubs tuned for other bands that coil might become a slight loading coil, making the attachment points of the stubs lower, or it may become a choke at higher frequencies degrading reception there.

The stub tuned multiband antenna plans were originally for starting with a 1/4 wave at the lowest frequency and you cannot use those stub dimensions when using stubs on a 5/8 VHF. You would have to start from scratch and model the antenna to find out if stubs can be used and where they would mount.

He does have a coil. Unfortunately, it is for the VHF 5/8 antenna in the picture.
Larry

 

[patmcc24]

The coil is gutted... it was low band 27Mhz.. i removed the winding a inner thread sleeve and replaced with a straight thru..... i actually tried it for different bands had to move the 800Mhz stub to bottom .... compared to the old cheap Radio Shack outdoor antenna I can pick up a control at 859.6625 that is 61 air miles on my 15x the signal bar runs 1 flashing 2.... hooked this up on a temp mount middle of kitchen and recieved steady 2 bars... I feel i am going in the right direction also i loaded a weather 164.200... i touched top center mast slid my finger down 6.25 inches and killed the recieve... not sure if thats good or bad... like i said its been years .... honestly would have went full wave... if i had a stainless whip just 20 inches longer.

 

[prcguy]

A full wavelength will not match to 50 ohms at all and the resulting pattern will have a null at the horizon. A 1/4 wave vertical will greatly outperform a full wave vertical even with the full wave properly matched.

Its not likely you will hit upon a combination of stubs and antenna lengths by random trial and error. It will end up in a lot of error and time spent spinning your wheels. Its better to design an antenna with known accepted practices, then model it, then build a test sample. What your doing is kind of like trying to increase engine horsepower by pouring random liquids in the gas tank, then trying to feel if the car accelerates better.

The coil is gutted... it was low band 27Mhz.. i removed the winding a inner thread sleeve and replaced with a straight thru..... i actually tried it for different bands had to move the 800Mhz stub to bottom .... compared to the old cheap Radio Shack outdoor antenna I can pick up a control at 859.6625 that is 61 air miles on my 15x the signal bar runs 1 flashing 2.... hooked this up on a temp mount middle of kitchen and recieved steady 2 bars... I feel i am going in the right direction also i loaded a weather 164.200... i touched top center mast slid my finger down 6.25 inches and killed the recieve... not sure if thats good or bad... like i said its been years .... honestly would have went full wave... if i had a stainless whip just 20 inches longer.

 

[Ubbe]

Half wave and full wave elements have very high impedances in the Kohm range at their tips and you have to use some sort of impedance transformer to get it down to coax impedance 50-75 ohm.

/Ubbe

 

[patmcc24]

A full wavelength will not match to 50 ohms at all and the resulting pattern will have a null at the horizon. A 1/4 wave vertical will greatly outperform a full wave vertical even with the full wave properly matched.

Its not likely you will hit upon a combination of stubs and antenna lengths by random trial and error. It will end up in a lot of error and time spent spinning your wheels. Its better to design an antenna with known accepted practices, then model it, then build a test sample. What your doing is kind of like trying to increase engine horsepower by pouring random liquids in the gas tank, then trying to feel if the car accelerates better.

Language i understand lol... ok so 1/4 will do better than what i have... i guess i can cut it down i didnt make anything permanent yet so i am open to anything... i know i could just spend 89.00 and have a nice Tram or something that would serve my purpose perfectly but you dont learn by unboxing and hanging.... one more thing i honestly do not need the 450 band ... can i eliminate that stub and place the 800 there?

 

[prcguy]

A 5/8 with some matching at the base will do a little better than a 1/4 wave, but it really complicates adding other bands with stubs or traps, etc. There seems to be existing plans on a multiband 1/4 wave stub antenna, so that should be easy to duplicate.

Language i understand lol... ok so 1/4 will do better than what i have... i guess i can cut it down i didnt make anything permanent yet so i am open to anything... i know i could just spend 89.00 and have a nice Tram or something that would serve my purpose perfectly but you dont learn by unboxing and hanging.... one more thing i honestly do not need the 450 band ... can i eliminate that stub and place the 800 there?

 

[W5lz]

A 1/4 wave's input impedance is fairly close to 30 ohm, sort of. A half wave is much higher, round a couple of hundred and higher. A 5/8 wave is also fairly lower than 30 ohms so a impedance matching coil at the base of that antenna is required. If you scale the existing 5/8' whip length up/down for additional bands then add a stub for another band (total length of the end of the stub to the antenna's base you'll get it into the ballpark. Check to see where it's resonant and adjust from there.

Hmm, not very plain is that. Oh well...