Antenna questions

[sirsmiley]

Im debating making a quarter wave antenna with the one center element and 4 lower radials...its a 0db gain omni antenna..does it perform the same as a dipole antenna (basic antenna with a balun) its a half wave antenna i think

i planned out a 6db jpole antenna but it wont fit in my attic

 

[digproc]

A few of points to ponder:

If its advertised as "0dB", then that means its strength in its strongest direction is the same as the strength of the strongest direction of a dipole. However, a horizontally-mounted dipole has a radiation pattern that is a donut on its side, which means there are null zones on the ends of the dipole, and also half its energy is sent toward the ground. A vertical half-wave with a ground plane is much better in that pretty much all the energy is sent where you want it--up and out in all directions and with a nice null directly overhead. Very little energy goes downward.

Ideally your antenna should be mounted at right angles to any other nearby (within a wavelength or two) wiring, like power lines going to light fixtures, cable TV cables, telephone lines, etc. This is another advantage of the vertical driven element (the horizontal elements are a ground plane and not driven, so they don't count as far as this issue goes).

You'd also want to consider the typical orientation of the other guy's antenna, unless the signal will be skipping off the ionosphere, anyway, which changes polarization willy nilly. Polarization is less of an issue with higher frequencies because they bounce around a lot more, which randomly changes the polarization, anyway.

 

[Murstech]

A grounplane is a better choice. If you droop the radials it will be 35 ohms wich is a good match for coax. Coax... Wich is also unbalanced like the groundplane. Good deal.. And Im almost sure the radiation pattern is better than a vertical dipole too.. That means better coverage.

 

[sirsmiley]

thanks ive got the parts already ill make one and on a cooler day ill venture into the attic to hang it

 

[benbenrf]

What freqeuncy/cies is this antenna goin to be made for and what are your antenna dimensions?

3dBi from a quater wave groundplane type antenna should be possible to achieve in an attic space (depending on centre frequency).

 

[W2NJS]

Ground Plane Antenna...

You would realize a theoretical gain of 3DB if you were to use a 5/8 wave antenna, not a quarter wave. The quarter wave has only unity gain.

On VHF the 5/8 would give better results in most cases than the quarter wave, but instead of just slapping
the thing together like you do with a quarter wave the 5/8 requires some kind of matching transformer at its base. If you shop around almost all of these types of antennas can be store-bought these days pretty cheap.

 

[sirsmiley]

im looking at 140-145mhz is 5/8 directional?

 

[Murstech]

A 5/8 wave is not directional. Its a better choice than a 1/4 wave but requires a matching network.

 

[sirsmiley]

ive looked all over various websites and they have length calulators but nothing showing a simple diagram on how to make a 5.8 vertical antenna... ! yikes....or how to make a matching transformer

 

[Murstech]

If your going to make something from scratch I stick with the 1/4 wave.

 

[RadioDaze]

A 5/8 will have a lower radiation angle than the 1/4... something to consider if you are in a valley. If your elevation is high, a 5/8 would be a good choice and keep the signal down closer to the horizon. And if you're lower down, and want to get some rise out of your signal to hit repeaters, a 1/4 wave is better. And of course easier to build.

 

[sirsmiley]

ok i guess ill stick with 1/4 wave ill make one tonight with pics (it wont be going in the attic tonight i dont think, ill wait for a rainy day!)

 

[nd5y]

ive looked all over various websites and they have length calulators but nothing showing a simple diagram on how to make a 5.8 vertical antenna... ! yikes....or how to make a matching transformer

One time I dissected a Larsen NMO-150 high band 5/8 wavelength mobile antenna to see what they used for a matching network.
It is just an inductor in series with the whip.

The coil was 4 1/4 turns of about 16 gauge wire 3/4 inch diameter and 3/4 inch long. The total radiating element length (from the tip of the whip to the top of the coil, including the mounting cone and threaded stud embedded in the coil housing) is 49 1/4 inches at 146 MHz.

Go to your library or someplace and find a copy of The ARRL Antenna Book or The ARRL Radio Amateur Handbook.
They show you how to make another version of the 5/8 wavelength whip for 2 meters that uses a 47 inch whip and tapped inductor matching network.
It is 10 1/2 turns of 10 or 12 gauge wire on a 3/4 inch diameter form. the base is grounded and the feed point tap is approximately 4 turns from the top.

 

[sirsmiley]

ill stick with the 1/4 wave as i live in a deep valley sucks...the town is in a giant valley with everything else up top

 

[benbenrf]

You would realize a theoretical gain of 3DB if you were to use a 5/8 wave antenna, not a quarter wave. The quarter wave has only unity gain.

On VHF the 5/8 would give better results in most cases than the quarter wave, but instead of just slapping
the thing together like you do with a quarter wave the 5/8 requires some kind of matching transformer at its base. If you shop around almost all of these types of antennas can be store-bought these days pretty cheap.

I belive I said dBi - not dB (your comment was in reply to my earlier note, or do I stand to be corrected?), in any event the 3dB gain you quote, if a 5/8's is used ........

This 3dB is versus what - an isotropic reference, a 1/4wave reference (i.e. dBd) or a 1/4 wave ground plane (i.e. 1/2 wave dipole) - and is this absolute or relative?

From my side the 3dBi figure I gave mean't 1dB (absolute) gain compared to a 1/4 wave reference (i.e. dBd)

On the question of which would be better - 5/8 or 1/4, well assuming space allowed for setting up a 5/8's for the given wavelength the OP has in mind, and keep in mind that a 5/8's versus a 1/4wave for a given wavelength (frequency) is going to be around 3 times longer, folk - yes, it will indeed offer higher peak gain, but but - that extra gain is no free lunch: expect the 5/8's t be more "peaky" than the 1/4wave i.e. the additional gain obtainable from a 5/8's will be offset by a narrower -3dB bandwidth, outside of which the 5/8's may well turn out to be a weaker performer than the 1/4wave.

As this is a scanner application, I think I'd prefer to sacrifice peak gain for better/wider/more spread out day to day -3dB bandwidth performance.

The problem of course is, despite asking sirsmiley to share with me the frequencies and/or bandwidth of interest to him, he has not as yet - so my comment (and opinion) re: use a 1/4wave as opposed to a 5/8wave is, like all the other comments made here (and on other "which is my better choice" kind of threads), really only be valid if the relivant criteria are taken into consideration - and in this case (amongst other considerations) the one consideration has to be: what is the proposed band width coverage the OP has in mind - 1 Octave, 2 Octaves, 500Mhz, 750Mhz .......?

Sirsmiley - what are your frequencies of interest, and how widely are they spread?

 

[RadioDaze]

I didn't see any indication that this was for receive only. I'd think a 1/4 wave should work fine - and he's in a valley, to boot.

 

[sirsmiley]

i made one tonight, drilled out the connector a little bit for the 4 holes so i could use heavy duty nuts and bolts...i used 14 gauge galvanized wire which was easy to work with (electric fencing wire) and i made the cuts approximately 50cm after the loop for the bolts.. (to work with low 140mhz) and a center vertical piece the same length , i soldered overtop of the four bolts making them super secure for the radials and soldered the center piece in....i havent put an so-239 adapter to bnc in with a cable yet...next week ill get a chance to climb into my attic for the first time in ages and see if i cant place it and run the cable down into my computer room...

 

[benbenrf]

140Mhz ........ well if space is the issue then a 5/8's is out of the question, but you're in a valley so don;t expect to get much out of the 1/4wave either. Your performance is going to largely be dependant on how well you can isolate this antenna setup an dhigh you can get - youre making the best out of not very helpful circumstances. The addition of a steep bandpass filter may well be worth thinking about. Yes, it may well lower your already weak gain figure, but the amount of noise a well designed bandpass removes often more than makes up for any loss it introduces. In short, the better your SNR figure the better this antenna is going to work.

Set it up as far as poss from any copper/steel boiler and/or water storage tank you may have in the attic.

Let us know how it works when set up - and good luck.

 

[sirsmiley]

with the attic ill only gain a few feet but its worth trying, im already on the second floor, theres nothing in my attic but lots of loose insulation that im sure will come flying out of the access hatch ill be bringing my respirator from work home before i go up there, ive never been up there for more than a very quick peek

 

[jackj]

Gentlemen dB is not a direct measurement of power, it is always a reference to another level of power. The referenced power is usually indicated by the subscript, dBm is referenced to 1 milliwatt. When measuring antenna gain, the reference is usually to an isotropic antenna (dBi). An isotropic antenna is a point source and has a radiation pattern that is a perfect sphere, an impossible pattern to obtain in reality. A 1/4 wave dipole will have somewhere around 2 to 2.5 dB of gain over an isotropic. So when quoting antenna gain you should list the reference otherwise your figure has no meaning.

Lecture mode off: Flame retardant suit is on: