1/4 wave or 5/8 wave - which might work better?

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br0adband

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I am interested in building 2 antennas specifically for CivAir and MilAir monitoring duties. As I live in an apartment in a building with no ability to get an antenna outside in most any respects I - like so many other apartment dwellers - am forced into finding indoor solutions of most any kind and hoping for the best. I've been toying with the idea of making a funky mount of some kind with clamps to get an antenna outside my window and uses the frame of the window itself as part of the mount but I haven't moved forward on that idea.

I was also thinking about building a discone cut for about 118 MHz on the low end but as an indoor antenna that would end up being rather large and the Wife would most likely have a problem with it herself. :)

Anyway, I've been doing research into different antenna types and I'm well aware of how a dipole works and the fact that it's used as "the reference" in terms of antennas and performance. I also understand how a 1/4 wave ground plane works (quite well in my experience) as I built one of those very simply homebrew versions using an SO-239 chassis mount, some screws and nuts, and coat hangers and so far it's worked better than I imagined it would. I haven't even soldered all the parts together and I have several elements cut for various frequencies that I just swap out as the main radiator whenever I'm interested. Typically I just leave the 150 MHz element in place (per the original design) and I haven't bothered to add the parasitical elements for 450/800 MHz per the original design which you can find at the link below yet I still get great performance from those bands when I scan them:

http://www.qsl.net/n4yek/scanner/antenna.pdf

But I am wanting to focus more on CivAir and MilAir nowadays. I see that "blade" design antenna by DPD Productions (link here) and while I know the owner of that company has a great reputation and I'd love to have one of those it's simply not possible at this time so I figured I'd try to emulate it with a homebrew version. According to the math, a 5/8 wave monopole for ~127 MHz would be 55.3 inches in length; the DPD blade antenna claims a length of 60" so I'm going to assume (oops, assumption incoming) that the element inside is a ~55 inch one allowing for the housing it's encased in to make up the rest of the length top and bottom (2-3" or so), or something very close to those dimensions.

Also, the specs note that there is gain of 2.6 dBi for that blade antenna, and while I'm not an expert on antenna design by any length, the research that I've done in the past few weeks points to 5/8 wave antennas actually having a slight bit of gain over a traditional dipole at the same frequency. If that's the case, and I make such a beast properly, I should expect to get better performance (another assumption) over making a standard dipole for ~127 MHz or even a 1/4 wave ground plane for that frequency as well.

Am I just nuts here or would someone that's far more knowledgeable about antennas correct me on that assumption if I'm wrong? I mean, again, I'd love to have a really nice commercially produced antenna, sure, and help keep people in business but right now that's not something I can do so, in the effort to try and squeeze out the best performance from what I have access to (wire, some pipes - got a 58" long 1" diameter piece of aluminum pipe that I'm going to try out here later, etc).

Also, is there a requirement - if I go the 5/8 wave route - to have a ground plane because I saw a design that suggests matching the 5/8 wave element with at least three 1/4 wave elements for the ground plane.

And lastly: with this particular setup, either with or without the ground plane, should/would I require a matching transformer or balun for using the coax feed line, or can I just attach the coax leads to the element(s) and that's that?

Yeah, I know I can just build this and be done with it and hope for the best but I figured hey, can't hurt to ask and see what advice or suggestions I get back.

Last thing: I'd be doing this same type of build cut to ~300 MHz as well, so... based on what's offered (if anything) for the 127 MHz variant I would assume (DOH!) that it would work exactly the same, just at the different design frequency... right? Right? :p

Thanks for any input, it's greatly appreciated.
 

davenlr

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You really do not need gain for aircraft, unless you are wanting to listen to them on the ground at the airport, and the airport is not line of site. A coaxial dipole would be sufficient.
 

LtDoc

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There are several things that are going to complicate things. An indoor antenna will never be as 'good' as the same antenna outside. Doesn't matter which brand/model, it's just a fact. Deals with walls/ceilings attenuating the signal.
If you can use a resonant antenna, one made specifically for some particular frequency, it'll be better than one not designed for that frequency. With just listening/scanning, antenna lengths aren't that critical. But, close does count.
If you can figure a frequency that's sort of the 'mean' of all of them you want to hear, you can use an antenna that's made specifically for that 'mean' frequency and it'll 'work' on most of them. I can't give you an example for that, I have no idea what you want to listen to.
There's no -one- antenna that will 'do' it all 'well'. Sorry 'bout that...
- 'Doc

If you are talking about listening to aircraft in flight, then a 1/4 wave antenna would probably do better than a 5/8 wave antenna. That's because a 1/4 wave typically has higher angles of radiation than a 5/8 wave does. Since aircraft are at 'higher angles' than land stations, then those higher angle thingys tend to work good.
 

br0adband

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... I have no idea what you want to listen to.
Very first sentence:

br0adband said:
I am interested in building 2 antennas specifically for CivAir and MilAir monitoring duties.
And yes I'm aware of the angle considerations, and it being inside, etc, I explained all that in my all-too-wordy manner in the first post, least I think I did. ;)

I'm also not on the ground, I'm 3 floors up so that helps to some degree, and another aspect of being in the near-dead-center of the entire Las Vegas metropolitan area is that we're surrounded by mountains so I would only be getting signals from aircraft once they cross over the LoS vantage point to my location above the given mountain ridges anyway.

You really do not need gain for aircraft, unless you are wanting to listen to them on the ground at the airport, and the airport is not line of site. A coaxial dipole would be sufficient.
That's pretty much the idea: listening to everything I can, of course. Nellis AFB is ~6 miles up the road northeast of me, McCarran International Airport is ~6-7 miles almost due south with a slight east angle to it, and the Las Vegas Airport (small local thing) is about 7 miles northwest of me, and again I'm central in the entire region so anything is better than nothing I guess. I don't expect to hear ground chatter from any of them but it can't hurt to try and improve things.

We'll see what happens I guess.
 

nanZor

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I suggest looking at this from a slightly different angle when wanting to do both civil and milair monitoring.

Get another scanner dedicated solely to milair. Reason being is that milair comms are so short, and by the time you've listened to two or more civil touch-n-go's at the local airport, the mil comms are over, not to be heard for another hour - even with the fastest scanner, this can be a problem when trying to listen to both.

As for antennas, just keep it simple like you are doing and don't overthink it at this stage. Build another chassis-mount groundplane for the milair scanner, but this time cut it for 300mhz (center of milair band more or less) so each element would be 9.25" inches long - radials and center element. OR build a vertical wire dipole each 9.25" long each side with #12 house wire or similar.

Like how I justified another scanner? Tell the better half that the RR guys said that this is the only way to go. :)
 
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br0adband

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I had the understanding that when making the 1/4 wave ground plane that the ground plane radials should be about 5% longer than the main radial, at least that's how that "classic" design from that PDF linked above has it in that respect (19" main radial at 150 MHz and then 20" ground plane radials - it can't hurt to do that with a 1/4 wave ground plane cut to 300 specifically I'm sure. But then again, if the tri-band antenna (which I still haven't actually built using the parasitic elements for 450/800) works, wouldn't it be possible to add a 300 MHz element to that too? :D

/me shoots the moon, just because he can...

As for scanners, well, I'm into the SDR kick nowadays and with my two RTL sticks + SDR-Radio I can actually monitor 12 frequencies at the same time (6 for each stick) given they fall inside the spectrum bandwidth I'm monitoring. I can even just use SDR# with the sticks as well, one dedicated for CivAir scanning (yep, it'll work but it's nowhere near the same as having a real actual scanner of course), one for MilAir, and so on.

Damn I really wish I could get a discone up on the roof of my building, I really do. :)
 

prcguy

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Newer information says to make the ground radials the resonant length and not 5% longer and the more radials the better. On a multiband antennas its best to have a set of radials for each band but they can work ok on odd multiples like 150MHz radials used for 450Mhz.

When you have more than one radial, radiation is canceled in the radials due to (hopefully) equal currents flowing in opposite directions. 1/4 wave long radials provide a low ground impedance where 1/2 wave or some arbitrary length can raise the impedance of the ground system.
prcguy


I had the understanding that when making the 1/4 wave ground plane that the ground plane radials should be about 5% longer than the main radial, at least that's how that "classic" design from that PDF linked above has it in that respect (19" main radial at 150 MHz and then 20" ground plane radials - it can't hurt to do that with a 1/4 wave ground plane cut to 300 specifically I'm sure. But then again, if the tri-band antenna (which I still haven't actually built using the parasitic elements for 450/800) works, wouldn't it be possible to add a 300 MHz element to that too? :D

/me shoots the moon, just because he can...

As for scanners, well, I'm into the SDR kick nowadays and with my two RTL sticks + SDR-Radio I can actually monitor 12 frequencies at the same time (6 for each stick) given they fall inside the spectrum bandwidth I'm monitoring. I can even just use SDR# with the sticks as well, one dedicated for CivAir scanning (yep, it'll work but it's nowhere near the same as having a real actual scanner of course), one for MilAir, and so on.

Damn I really wish I could get a discone up on the roof of my building, I really do. :)
 

nanZor

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Damn I really wish I could get a discone up on the roof of my building, I really do. :)
Hope the front-end of your SDR gear can handle it. That is, a discone is great for providing a very good impedance match over a wide range of frequencies, but in a metro area that could also lead to out of band intermod, desense and the like requiring you to put bandpass / notching filters inline. That can make for an expensive catch-22. :)

In this case, a simple dipole or groundplane tuned to center frequency of the band of interest may prove to be the better performer.

prcuy is right about the lengths. For receive-only, you won't notice a big difference with a little sloppiness at these freqs. Build one with an inch longer than normal. Then build one with perfect dimensions. Do a blindfold test on rx. You'll be hard pressed to notice a big difference.
 

verbatimx

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Dec 3, 2007
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But I am wanting to focus more on CivAir and MilAir nowadays. I see that "blade" design antenna by DPD Productions (link here) and while I know the owner of that company has a great reputation and I'd love to have one of those it's simply not possible at this time so I figured I'd try to emulate it with a homebrew version. According to the math, a 5/8 wave monopole for ~127 MHz would be 55.3 inches in length; the DPD blade antenna claims a length of 60" so I'm going to assume (oops, assumption incoming) that the element inside is a ~55 inch one allowing for the housing it's encased in to make up the rest of the length top and bottom (2-3" or so), or something very close to those dimensions.

Also, the specs note that there is gain of 2.6 dBi for that blade antenna, and while I'm not an expert on antenna design by any length, the research that I've done in the past few weeks points to 5/8 wave antennas actually having a slight bit of gain over a traditional dipole at the same frequency. If that's the case, and I make such a beast properly, I should expect to get better performance (another assumption) over making a standard dipole for ~127 MHz or even a 1/4 wave ground plane for that frequency as well.

Am I just nuts here or would someone that's far more knowledgeable about antennas correct me on that assumption if I'm wrong? I mean, again, I'd love to have a really nice commercially produced antenna, sure, and help keep people in business but right now that's not something I can do so, in the effort to try and squeeze out the best performance from what I have access to (wire, some pipes - got a 58" long 1" diameter piece of aluminum pipe that I'm going to try out here later, etc).

Also, is there a requirement - if I go the 5/8 wave route - to have a ground plane because I saw a design that suggests matching the 5/8 wave element with at least three 1/4 wave elements for the ground plane.
Hi br0adband,

I think that DPD blade antenna has inside a j-pole design because it do not need ground plane as 5/8 antenna design.

Check this link 2 METER LADDER LINE J POLE PROJECT- BUILD IT FOR A SONG!.

Greettings
 
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