ATTENTION ALL EXPOSED FOLDED DIPOLE EXPERTS

[FF/EMT/KC2NFB]

im looking to move the elements of this antenna from the stock collinear position to an omni position. i want to lower the gain and even out the pattern.

my questions are:

1- will the antenna function properly?

2- what will the gain be?

my educated guess is that the gain will be around 5 dbd.

here's the spec sheet
http://www.andrew.com/products/antennas ... -pg341.pdf

thanks for the help.

[rodell]

I believe it would be an omnidirectional at 6dB, give or take. This would be if you set the dipoles 90 degrees apart from the previous one in the same direction as you continued up (or down) the mast. (Like the db224 - http://www.andrew.com/products/antennas ... 224-C.aspx )

I don't see the distance from the mast listed - that would also play a role.

Rob

[FF/EMT/KC2NFB]

the db-224 is what i based my guess on.

thank you

[Dan562]

For what want to do to the 4 folded dipole elements would create a +6 dB Omni Directional Pattern. Don't worry about the distamce between the folded dipole elements to the vertical mast pipe.

Dan

[Birken Vogt]

That's interesting, there are some very sharp nulls either way you point it from or away from the tower. The DB224 stays pretty even by comparison.

Birken

[440roadrunner]

Rather than ask us here, I'd be tempted to get ahold of whoever makes whatever antenna you have.

I don't know how you can HELP but worry about the distance to the tower. It simply MUST be relevant

[tvsjr]

That's interesting, there are some very sharp nulls either way you point it from or away from the tower. The DB224 stays pretty even by comparison.

Birken

First, it's a UHF antenna, so it's a bit different from a 224. But, it's also a "quasi-omni pattern". A DB408 it ain't.

[Birken Vogt]

I didn't even know it existed until I saw this, I thought the 404 was all there was.

Birken

[tvsjr]

Not at all. The 404/408/420 are the classic dB folded dipoles for UHF. I want to say the 411 came from one of Allen Telecom's other acquisitions (Cushcraft, maybe? I can't keep them all straight).

[FF/EMT/KC2NFB]

thanks for all your input. i sent a tech question to Andrew ill let you know what they say.

anyone else have anything on this subject please post it.

thanks

[Will]

A much newer design and better antenna...


http://www.txrx.com/product/product_vie ... 6FFE3909CB

[RFdude]

You will notice from the TXRX antenna someone else posted that for true omni, there is double the number of dipoles. Sinclair makes a SRL-335 and SRL-335-2 that has been in service many years.

However, my preference for simplicity is to stick to the "offset" or "bi-directional" pattern (such as the Sinclair SRL-310-C4 (4 dipole, 8 dBd) and design the offset into the coverage. For VHF, I only use 210-C2 (5.5 dBd) since this band seems to be band limited. Half wave spacing is most common for me since it is closest to omni. Really, when comparing to a true omni like a 335, the extra cost, complexity, double # dipoles and external harness, etc... it seems useful to me only for power handling.

By the way, look at the COMPROD antennas.. they are very good quality. Come in very robust Heavy Duty versions too.

Another nice thing about any folded dipole antenna is that the mast makes a very good lightning rod. When ordering, be sure to request the lightning spike for the top of the mast.... 18" is usually a good length to protect the top most folded dipoles.

I've seen a few installations where they have put this 335 type omni antenna side mounted to a tower. Don't know why this would be done... perhaps someone could expand on this.

RFdude

[Will]

Most of the hundreds of antennas used at the GOOD managed sites, are the dipole offset style.

Mine are the Sinclair 9db offset four dipole UHF antennas put up many years ago. They kick democrat!!!!!

A couple of sites here require a deep null to the back side to protect our neighbors. So the dipole type work really well.

[Birken Vogt]

When you guys say "offset", are you referring to the folded dipole that resides out on a long arm, say a foot or more long, as opposed to the close in ones such as the DB224?

Birken

[Will]

Birken, the "offset" refers to the dipoles on one side of the mast creating an offset or Cardioid pattern. The spacing from the mast to the dipole determines the depth of cardioid pattern on the opposite side of the mast, a lower gain thus a notch to the "backside". As the dipole to mast spacing is changed the shape of the cardioid "notch" changes.

Also the spacing of the dipoles from the mast determines the overall forward gain especially at the + and - 90 degree point. The deeper the backside notch the slightly more forward gain.
This can be one db on a four dipole antenna.

[RFdude]

In a nutshell, the 1/4 wave spacing (dipole to mast) produces the near circular offset pattern with the least amount of gain in the direction of the mast. It really isn't a null as one would expect from a directional antenna. The front to back ratio if you could call it that is 2 to 7 db depending on the number of dipoles in the antenna. For instance, a single dipole antenna is almost omni.

For 1/2 wave spacing, the pattern becomes more like a peanut shape with the highest gain left and right (90 degrees) to the dipole arm. In effect there are two directions where the gain is highest and thus it is called bi-directional.

However, unlike a directional antenna (yagi, corner reflector, flat panel, etc), the differences are only a few dB. You best have a look at the antenna patterns that can be found on-line at your favorite antenna manufacturer. For example, page 15 of http://www.comprodcom.com/english/pdf/b ... 5-Full.pdf

RF Dude

[Birken Vogt]

I saw something I didn't know about before in that pdf...the dipoles with the reflector behind them will really fill the bill for a couple of projects I have in mind. Around here the mountains with the best elevation aren't always right in the center of your coverage area but that almost half-moon pattern will do very nicely.

Birken

[Will]

Around here the mountains with the best elevation aren't always right in the center of your coverage area but that almost half-moon pattern will do very nicely.

Birken

It IS that way in California. We have the same thing in So. Cal.