diversity antennas

jim · Post by **jim** » Sun Dec 09, 2001 7:44 pm

What is the proper formula for calculating the spacing between 2 UHF 1/2 wave antennas for a true diversity setup? How about the lengths of the cables to the divider?

radioconsult · Post by **radioconsult** » Mon Dec 10, 2001 12:13 pm

First are you planning on connecting two base antennas togather with a tee type connector and running this to one receiver? If so, this isn't diversity reception. True diversity is a special receiver with two input paths that either sums the signals or selects the better signal. If you are planning on using two antennas and one receiver input, all variables contribute to the pattern and gain. In some directions you may experience an increase in signal strength while in another direction you may see a decrease. It all depends on the spacing and the length of each jumper. Not recommended unless you don't have any other choice. Diversity separation is normally 10 wavelengths.
RC

jim · Post by **jim** » Mon Dec 10, 2001 1:52 pm

Ooops! I really meant a "co-phased" type- not diversity which analyzes the signals. It's going to be 2 UHF's to form a figure 8 type patter to cover an area thats about 10 miles by 45 miles. The antenna set will be centrally located.

Will · Post by **Will** » Mon Dec 10, 2001 6:03 pm

Two OFF-SET dipole antennas on oposite sides of the mast or tower is what I think you mean to cover the long narrow area. A very good idea to improve coverage. Combining two antennas with the proper impedance matching results in twice the antenna capture aera and twice the efective radiated power minus the matching losses. Fairly close to the 3 DB gain or twice power.
To connect these two antennas you need a phasing harness which is esentially a power splitter/combiner made from tuned lenths of 75 ohm cables and "t'd" together to get the 50 ohm feed point for the radio, like a transformer. The lenth of the 75 ohm cable is critical and of course varies with frequency. You may use 50 ohm coax or hard line to connect the combining harness legs to the antennas. Some exposed dipole antennas that use two, four or eight dipoles have "phasing" or matching/combining harnesses to combine the dipoles in pairs. A four dipole antenna uses three matching harnesses, one for each pair of dipoles and one to combine the two pairs' matching harnesses into one feed point.
Antenex makes them as does DP Products, Maxrad and others.

jim · Post by **jim** » Mon Dec 10, 2001 8:31 pm

I got it now! I don't do that much with this type of base antenna system. I
found the Antenex harness. My question now is......what do I go by for
spacing of the two antennas? I can't seem to find info on this anywhere. I'm
using two Antenex FG4605's. I know there some simple math for this, but can't
find it.

thanks all

raymond345 · Post by **raymond345** » Tue Dec 11, 2001 2:41 pm

In Canada, Canadian Railroad uses
many times, 2 yagi antennas 180 degrees apart
on the tower, a 50 ohms co-phased harness,
into their repeater with great vswr readings and a great signal along the tracks.]

Raymond eMail ve3hae@rac.ca

RFdude · Post by **RFdude** » Tue Dec 11, 2001 7:34 pm

In micro-cell applications for serving a road, a 180° arrangement is often used. As the previous lister wrote, the railways have a similar application where UHF is used and they have straight line interests. Keep in mind that the panel antennas have a F/B ratio that is 20 dB or better, so there is little interference between the two antennas. The side lobes that interfere with each other aren't important as that coverage is unintentional. You do LOOSE 3 dB for the 50/50 splitter however.

In your application as you are still looking for 10 miles WIDE coverage, it sounds like you can create a mess with improperly phased antennas. Go look at http://www.kathrein.com/ where they describe various patterns, phasing harneses, and spacing for their directional antennas at VHF and UHF. It is good insight. This company makes broadcast arrays so they are well versed in this area. They are very expensive. Especially at VHF, controlled patterns and high antenna gains aren't cheap.

I think that after all is said and done, AND considering your 3 dB splitter LOSS, you may be best off with a folded dipole arrangement using half-wave spaced dipoles. This will give you the "bi-directional" coverage you desire. Example: Sinclair 310-C4-2 or go 310-C8-2 if you need even more gain. At 800 MHz, there are equivalents ..SRL 411 series. Other vendors have equivalent antennas. Good Luck!

Will · Post by **Will** » Wed Dec 12, 2001 12:56 pm

The FG4605 antennas are very poor antennas, a better option would be a YDA4501 dipole(cost) or Telewave ANT450D folded dipole (much better made, but costs) on each side of the mast or tower at the same elevation. We have even used this type of setup with two dipoles at 90 degrees for a pie shaped coverage area.
We have used the Telewave ANT450D single folded dipole with its adjustable spacing from the mast to change the pattern on a lot of applications with tremendous results. A very rugged and weather proof antenna, especially near the ocean at a airport. The Telewave will not corrode like the aluminium antennas, FG series and YFA series including the equivalants from Maxrad, ect;..
By the way I have had some real serious problems with the Antenex FG450x /FG460x series antennas not tuning up, causing severe intermod, and high VSWR, especially when used on a repeater, had to change all of them out. I use the Comtelco BS450U-C omni with a thousand times the performance and NO SWR!!!, no intermod!!! A much better choice for a repeater.
PS the Sinclair 310 series KICK ASS, but COST a hell of a lot more!

<font size=-1>[ This Message was edited by: Will on 2001-12-12 16:03 ]</font>

raymond345 · Post by **raymond345** » Fri Dec 14, 2001 2:11 pm

HI Will:
The comtelco BS450-U-C antenna.
Where can we get info on it?
I live in Canada and 85% of antennas are
Sinclair units that we install. Would like good ant. at a cheaper price.
Thks Raymond eMail ve3hae@rac.ca

jim · Post by **jim** » Fri Dec 14, 2001 2:41 pm

Thanks, Will!

I'll look into using the antenna you recommend.

Will · Post by **Will** » Fri Dec 14, 2001 3:12 pm

Raymond,

Comtelco Industries Inc. is in Glendale Hights, IL 800-634-4622

RFdude · Post by **RFdude** » Fri Dec 14, 2001 7:21 pm

You may also want to try COMPROD out of Montreal. Very similar design with the folded dipoles. Heavy duty versions too with 3/4" diameter dipole tubing where icing or wind is a factor. These antennas aren't that expensive... a 4 dipole UHF is around $500 US minus your company's discount. The rigger costs double that amount to replace a bad antenna. Money spent on a decent antenna isn't a bad thing! Do it only once and do it right!

Post by **wavetar** » Fri Dec 14, 2001 7:46 pm

The rigger costs double that amount to replace a bad antenna. Money spent on a decent antenna isn't a bad thing! Do it only once and do it right!

This is the reason we use Sinclair exclusively. I've never had to replace a 2xx/3xx/4xx series Sinclair antenna, and have also never seen another manufacturer's work better, although a few others are comparable in performance, but not in durability.

Todd

raymond345 · Post by **raymond345** » Fri Dec 14, 2001 8:59 pm

Hi Wavetar.

In 25 years we have had to change many
sinclair ant. lightning can get all not just ice.

Post by **wavetar** » Sat Dec 15, 2001 1:28 pm

Yes, I suppose lightning would certainly do it, but I've personally never seen an antenna take a lightning strike here in Eastern Canada. I've spoken with several techs with many more years experience working out here than I, and they it's pretty rare, only once or twice in the 20-30 years they've been in the field.

raymond345 · Post by **raymond345** » Sat Dec 15, 2001 2:48 pm

Well in Ontario this is common.
Raymond

RFdude · Post by **RFdude** » Sun Dec 16, 2001 6:48 pm

Lightning.... One Ontario province wide VHF system uses DUAL SRL-235-2 on top of their towers ... it adds about 50' to the tower and the base of the mast is 8.625"! This antenna has a total of 16 folded dipoles. There is a 1 foot lightning spike on top. Despite the rod, after 10 years of service, the top two dipoles have about 50 arc marks on them. Of these, about 7 to 10 will show some significant heat stress and perhaps about 3 or 4 holes where lightning has burned through. Usually the antenna return loss is still good, but the IMD performance begins to suffer for obvious reasons. The poor IMD drives the replacement, otherwise they still radiate. Not bad...

Post by **/\/\y 2 cents** » Mon Dec 17, 2001 12:06 am

Most of the time if lightning hits your antenna itself it is no good....especially if it is fiberglass.it will split it into a thousand pieces..Im talking about so. Floida where we are the lightning capital of the US..so I could be wrong.

raymond345 · Post by **raymond345** » Mon Dec 17, 2001 7:17 pm

Hi RfDude.

Yes i know the system very well.Yes and i have seen the repairs to the antennas.

Yes the antenna is climbed by a person and he changes a loop when it is bad.Climbing
the antenna mast is not my idea of a thrill.
Each to their own.