Fan vertical dipole

[PluckyPleco]

Hello,

I've got one listening position in my house that I can't easily get a coax run from an outside antenna to.

The good news is that it is just on one side of a stairwell so I can get a nice vertical dipole up on the back of one of the stairwell posts.

So the question becomes:

"Should I just go with a vertical dipole at the lowest frequency I want to scan?"

-or-

"Should I go with a fan dipole with different lengths cut to the different major bands I want to scan?"

Plec

 

[wesct]

i am not sure what a "fan dipole is".
a discone antenna has elements for each bamd. maybe that is what your talking about?

some points about antennas

you want as much height as possible (for line of sight)
and feed the antenna with good quality coax (lmr 400, 9913etc)
keep the length short and to the point (if you need 50 feet, dont buy 100)
when possible, try not to use adapters or barrel connectors
(1db loss/adapter)
dont use preamps unless your far away (front end overload)

hope this helps

wesct

 

[PluckyPleco]

Fan dipoles / discone definition

i am not sure what a "fan dipole is".
a discone antenna has elements for each bamd. maybe that is what your talking about?

Thanks for your response! I've already covered the normal height/feedline issues covered.

A discone has elements that are all the same length (well, one length for the disk and one for the "cone"). It's broad-banded-ness comes from it acting sort of like a transformer.

http://www.northcountryradio.com/Articles/Figures/dis1.gif
http://www.northcountryradio.com/Articles/discone.htm

A fan dipole is a set of dipoles for different frequencies all tied together at the feedpoint.

In my case I'd cut a dipole for about 43 MHz and then another one for say... 153 MHz.

The idea is that each of the dipoles would dominate the others at frequencies near its resonance.

I think I'll cut a single long dipole and then add the second, shorter one to it with alligator clips, if it improves my VHF-high reception, then I'll make it permanent...

Plec

 

[TeRayCodA]

Fan dipole,as in a scantenna?

 

[PluckyPleco]

Scantenna == fan dipole?

Fan dipole,as in a scantenna?

I don't have a scantenna to analyze, but that's what I'm betting. A glorified fan dipole.

Must say I'm a bit curious about its 300 ohm feedpoint impedance... It would be fun to throw on on an antenna analyzer and see that it REALLY looks like.

In general there is a trade-off, the bigger the antenna the more signal it can gather, but if you don't get the geometry right some of the received signal cancels out due to being out of phase with signal coming in from a different part of the antenna.

I'm NOT at all sure how much of the transmit theory applies on receive.

Plec

 

[wesct]

i understand now. the channel master 5094 look alike.

the last time i checked, i never found a resonant point anywhere- but it seems to work.

over the years, i have sold hundreds of those 5094's to alot of happy customers. the scantenna is the closest to it.

wesct

 

[wesct]

i understand now. the channel master 5094 look alike.

the last time i checked, i never found a resonant point anywhere- but it seems to work.

over the years, i have sold hundreds of those 5094's to alot of happy customers. the scantenna is the closest to it.

wesct

 

[kb2vxa]

Hi guys,

"I don't have a scantenna to analyze, but that's what I'm betting. A glorified fan dipole."

Glorified not, sloppy yes but it works and that's the main thing. It's equivelent to a discone receive wise but more compact so if you're side mounting an antenna a Scantenna would be the way to go, that's how the local PD has theirs mounted on the tower. BTW, they have a discone on the substation along the boardwalk, their radio techs seem to know their stuff.

 

[PluckyPleco]

It's equivelent to a discone receive wise but more compact so if you're side mounting an antenna a Scantenna would be the way to go,

I've read about the Scantenna being better than a discone. Hype?

I can see a couple of possibilities...

A discone is only REALLY effective over a 3:1 range from its low freq. The vertical lobes seem to come off the horizon at higher frequencies, so if the observer was at a greater than 3x frequency range the discone would be spotting the dipole maybe 10 to 15 dB on the horizon. That would make a big difference.

But from within the discone's frequency range? It may spot the dipole 3 dB but not much more than that... What 1/2 an S-unit? That's gonna be hard to tell...

Plec

 

[kf4lne]

Remember to run your coax as close to 90 degrees to the antenna (vertical dipole means horizontal coax) for as far as you can. I would suggest creating several "elements" for the dipole using zip cord or twin lead or such or just plain wire. cut a dipole for ~40MHz, another for ~150MHz, another for ~450MHz and another for 800MHz. Translation: Cut 2 wires about 60 inches long, 2 about 20 inches long, 2 about 7 inches long and 2 about 4 inches long. You may also try a ground plane antenna so you get better feedline decoupling. I have a home brew antenna that I am using for a WX receiver antenna that tunes out at 146.500, 446.200 and has 800/900MHz elements on it. It worked great as a scanner antenna before it was replaced with my current antenna array. Ground plane antennas do NOT have to have more than 2 ground plane radials to operate properly. The important thing to remember is that antennas that are RESONATE will receive better than just an antenna so when you build your antenna remember to cut the elements to a length that are resonate at the frequencys you want to listen too. The easiest way to do that is with this formula: 234 / MHz = leg length, so to cut your elements for 150 MHz you would do 234/150=1.56, so you would have 2 wires for your dipole that are about 18 inches long to receive VHF-HI and the same applies to the rest of the elements.

 

[PluckyPleco]

More fan dipole advice

I would suggest creating several "elements" for the dipole using zip cord or twin lead or such or just plain wire.

_Precisely_ what I was planning on using.

cut a dipole for ~40MHz, another for ~150MHz, another for ~450MHz and another for 800MHz.

Due to my geography I only need to worry about VHF-lo and VHF-high. ;-)

The important thing to remember is that antennas that are RESONATE will receive better than just an antenna so when you build your antenna remember to cut the elements to a length that are resonate at the frequencys you want to listen too.

Cool, this is the answer I was looking for. It also sorta says that I want as large gauge wire as possible to expand the resonate range of the antenna.

Thanks & 73 guy,

Plec

 

[unitcharlie]

I understand the Channel Master 5094 has been discontinued.... does anyone know where I can get one--purchase timing depends upon Purchase Order approval and other factors beyond my control.... (I prefer the 5094 but will buy a 5094A, if I can find someone selling it...)

 

[TeRayCodA]

Keep an eye out for a 5094(A) on ebay,I have seen a stray that someone found while cleaning out their garage go for $60.00 (NOS,in box)at times.I picked one up about 15 yrs ago for $20.00 from a electronics store liquidation sale.I recently retired it,after replacing the VHFlo/main/top upper element a few years back from a thunderstorm mishap.Using some aluminum from a discarded TV antenna.
They are about the same lightweight TV antenna tubing as the scantenna.(But they were a Great antenna-5094!)

I found Radio Shack was clearing them (scantenna)out at $5.00! I called the district,and they had me one when I got to a store ,50 miles away.

Yes,the scantenna is the generic equal to the 5094.

 

[kb2vxa]

Hi Plec and all,

"I've read about the Scantenna being better than a discone. Hype?"

More hype than in a doper's works.

"I can see a couple of possibilities..."

Uh oh, here it comes. (;->)

"A discone is only REALLY effective over a 3:1 range from its low freq. The vertical lobes seem to come off the horizon at higher frequencies, so if the observer was at a greater than 3x frequency range the discone would be spotting the dipole maybe 10 to 15 dB on the horizon. That would make a big difference.
But from within the discone's frequency range? It may spot the dipole 3 dB but not much more than that... What 1/2 an S-unit?"

Pure gobbledeygook. A discone has unity gain over it's entire frequency range (bandwidth) meaning no funky vertical takeoff angles. Consider the basic fact it acts like an extremely broad band vertical dipole or groundplane.

"That's gonna be hard to tell..."

I just told it, end of story. (;->)

Post edit:
I rather got lost in all that dissertation, amend that to "within it's frequency range". Like all antennas, manufacturers tend to exagerate, they tend to extend the frequency range and/or frequency coverage within it's range without telling you "it just doesn't work well there".

 

[PluckyPleco]

I see you a goobbledeygook and raise you a fooey!

"A discone is only REALLY effective over a 3:1 range from its low freq. The vertical lobes seem to come off the horizon at higher frequencies, so if the observer was at a greater than 3x frequency range the discone would be spotting the dipole maybe 10 to 15 dB on the horizon. That would make a big difference.
But from within the discone's frequency range? It may spot the dipole 3 dB but not much more than that... What 1/2 an S-unit?"

Pure gobbledeygook. A discone has unity gain over it's entire frequency range (bandwidth) meaning no funky vertical takeoff angles. Consider the basic fact it acts like an extremely broad band vertical dipole or groundplane.

I call fooey on your gobbledeygook. ;-)

Yes, unity gain over its frequency range... But as a ham, I'll point you to the discone section of the ARRL Antenna book where there are elevation patterns for discones. When used at too high a frequency, the SWR remains good but the main lobes are up near 60 degrees. Most people don't have lots of scanning targets 60 degrees up...

Although there is a small "my bad", the patterns show only -3 dB on the horizon rather than 10 to 15 mentioned above.

A REAL plus of properly-sized discones is that they have a VERY sharp cutoff at their low frequency. If you homebrew one for 400 to 1.2 GHz, you don't have to worry about broadcast station overload/desense of your receiver. The discone acts as a natural high-pass (well... really band-pass) filter.

To do it right would require 3 discones: 50-150 MHz, 150-450 MHz and 400 to 1.2 GHz and automatic frequency-based antenna switching. (or three receivers) Oh BOY!

73,

Plec
In real life I sport a 2x2 call.

 

[kf4lne]

hmmm, I wonder who he could be....


And in real life I sport a 2x3 and at one point in time I got to sport a 4 letter call and 10kW for 6 hours a day until teh next guy came in

 

[PluckyPleco]

Final results:

Trimmed two pieces of zip cord.

One for VHF-Lo at 5.5' (times two)
One for VHF-Hi at 18.5" (times two)

I soldered them to a UHF Bulkhead mount and am feeding it with 3' of RG-58.

It is in a vertical dipole configuration.

On VHF-lo I have picked up one bar of signal on my 780XLT beyond what the homebrew 2m LEOSat antenna I had been using gave me. (And the antenna on the back of the radio doesn't hear at all).

My VHF-hi seems the same (the 2m LEOSat antenna did have SOME gain) but I haven't heard the specific signal I cut it for yet so it's hard to tell.

Using the handy-dandy antenna analyzer...

The VHF-Lo antenna is resonant from 42.2 MHz to 44.5 MHz.
The VHF-Hi antenna is resonant from 150.4 to 153.6 MHz.

(Resonant being pure real resistance without any complex component)

Now, if I can only catch a nice long transmission from the 151 signal I'm looking for to do an A/B test...

I'm ALMOST tempted to throw on a 1/4 wave stub and add another 1/4 wave or two for the VHF-Hi part.

Plec