Multiple antennas for multiple bands

[fmulder13]

Hey all,

I know that connecting two antennas to your scanner at the same time will result in half the signal loss. Now is that because of the t-connector or because of the idea the signals will reach the two antennas at different times, get to the scanner at different times, and thereby attenuate the signals? If the latter is true, would it be possible to cheat this? The idea I'm thinking of is mounting, say, a 5/8 wave VHF antenna and an 800 mhz antenna right next to each other. The theory behind my idea is that the antennas will be so different in resonance from each other that the 800 mhz antenna won't pick up the vhf signals, and therefore won't attenuate the signals received from the vhf antenna itself, and vice versa. I'm also figuring that if either antenna picks up signals from the other band, the signals would reach the antenna at roughly the same time, since the antennas are placed next to each other, and the coax lengths will be the same. Am I out of my mind?

 

[N_Jay]

When you hook multiple antenns together you are creating a complex network.

Unless you understand how to take into account the impedance of the elements (antennas, receivers, and junctions) and the effect of tat impedance transferred along a transmission line, you will get what look to be random results.

 

[Don_Burke]

I know that connecting two antennas to your scanner at the same time will result in half the signal loss.

Actually, that is not true.

Now is that because of the t-connector or because of the idea the signals will reach the two antennas at different times, get to the scanner at different times, and thereby attenuate the signals?

There are several forces at work here. Basically, you have added another place for the signal coming down cable "a" to go and that would be up cable "b". I can tell you that you are going to lose some, but putting a figure on it is beyond what we can figure from the facts at hand.

If the latter is true, would it be possible to cheat this?

It depends on the installation.

The idea I'm thinking of is mounting, say, a 5/8 wave VHF antenna and an 800 mhz antenna right next to each other. The theory behind my idea is that the antennas will be so different in resonance from each other that the 800 mhz antenna won't pick up the vhf signals, and therefore won't attenuate the signals received from the vhf antenna itself, and vice versa.

Couple them up at the antenna and most of the problems go away. The impedance of an off resonance antenna is pretty high and greatly reduces that antenna's effect on the other signal. Use the money you would have spent on two cables to get one really good one.

I'm also figuring that if either antenna picks up signals from the other band, the signals would reach the antenna at roughly the same time, since the antennas are placed next to each other, and the coax lengths will be the same.

See above

Am I out of my mind?

That is beyond the scope of this discussion.

 

[N_Jay]

. . . .Couple them up at the antenna and most of the problems go away. The impedance of an off resonance antenna is pretty high and greatly reduces that antenna's effect on the other signal. . . . .

Not necessarily, it could also be very low, almost a short circuit and therefor have great effect.

 

[Don_Burke]

Not necessarily, it could also be very low, almost a short circuit and therefor have great effect.

Name an antenna that does that.

 

[N_Jay]

Name an antenna that does that.

A folded dipole will show near zero at many frequencies outside its designed band, as will many antennas using transmission line transformers for matching.

As for other antenna design with L/C matching networks you have to look at the specific network design.

 

[Don_Burke]

A folded dipole will show near zero at many frequencies outside its designed band, as will many antennas using transmission line transformers for matching.

As for other antenna design with L/C matching networks you have to look at the specific network design.

So you are figuring a narrowbanded antenna like a folded dipole is going to be hooked to a scanner in parallel with a 5/8 wave?

How many transmission line matching transformers have you seen for scanner use?

L/C matching networks are so rarely used for broadband antennas I am surprised they even came up here.

Granted, looplike antennas go low off resonance, but their use in casual scanning is so rare I do not consider them worth bringing up.

 

[trooperdude]

There has been a pretty extensive thread on this.

Do a search for "diplexer"

 

[N_Jay]

So you are figuring a narrowbanded antenna like a folded dipole is going to be hooked to a scanner in parallel with a 5/8 wave?

How many transmission line matching transformers have you seen for scanner use?

L/C matching networks are so rarely used for broadband antennas I am surprised they even came up here.

Granted, looplike antennas go low off resonance, but their use in casual scanning is so rare I do not consider them worth bringing up.

You say keep the cable short, but 1/4 wave turns an open into a short, and 1/4 wave at UHF and 800 MHz is not much cable.

What I am saying is you are more likely to get poor results than good results, and if you want consistent good results you need a deeper education than you are going to get on an Internet forum.

Yes, if you want to do "multiple antenna's", just make a ground-plane with multiple vertical elements all connected a single point.

THEN your generalization will almost always be true.

 

[Don_Burke]

You say keep the cable short, but 1/4 wave turns an open into a short, and 1/4 wave at UHF and 800 MHz is not much cable.

Where did I say that? Are you mixing threads?

What I am saying is you are more likely to get poor results than good results, and if you want consistent good results you need a deeper education than you are going to get on an Internet forum.

Spending a couple of months chasing down education to make an educated guess at what you can find out with a day of experimentation does not strike me as a good idea.

Yes, if you want to do "multiple antenna's", just make a ground-plane with multiple vertical elements all connected a single point.

THEN your generalization will almost always be true.

Gee, that sounds _exactly_ like what the OP wanted to do.

Why did you find it necessary to make it so complex?

Next time bring up waveguides and beverage antennas. That will complete your mastery of off-point trivia.

 

[N_Jay]

Where did I say that? Are you mixing threads?Spending a couple of months chasing down education to make an educated guess at what you can find out with a day of experimentation does not strike me as a good idea. Gee, that sounds _exactly_ like what the OP wanted to do.

Why did you find it necessary to make it so complex?

Next time bring up waveguides and beverage antennas. That will complete your mastery of off-point trivia.

I (mis)interpreted you "connect at the antenna" to cabling them together close to the antenna.

maybe just a brain-fart. :roll:

But going back to one of my points, a 5/8 wave antenna DOES need a matching network of some kind, and could be a short at 800, just as easily as an open.

 

[Don_Burke]

But going back to one of my points, a 5/8 wave antenna DOES need a matching network of some kind, and could be a short at 800, just as easily as an open.

Nope, it gets _really_ high.

You will find that the "open" antennas tend to get more open off resonance and the "looplike" antennas get more like a short. There are some responses at harmonics and subharmonics, but fortunately they are not much of an issue for most scanning.

The glaring exceptions to this are things like gamma match / j-pole / shunt feed. (Which are not "open" or "looplike" anyway.) Those seem to be all over the place.

 

[N_Jay]

Nope, it gets _really_ high.

You will find that the "open" antennas tend to get more open off resonance and the "looplike" antennas get more like a short. There are some responses at harmonics and subharmonics, but fortunately they are not much of an issue for most scanning.

The glaring exceptions to this are things like gamma match / j-pole / shunt feed. (Which are not "open" or "looplike" anyway.) Those seem to be all over the place.

OK, lets tell teh OP to go have fun and experiment.

AND when he gets what seems to be random and unexpected results you will have a slightly better idea why.

Of course, as discussed in another thread, most don't have the understanding and equipment to do any meaningful tests.

You combine this all together and the "experimenter" is more likely to develop myths and superstitions than any type of education.

 

[Don_Burke]

OK, lets tell teh OP to go have fun and experiment.

AND when he gets what seems to be random and unexpected results you will have a slightly better idea why.

Agreed. It is _much_ better to send people off with guidance like this:

Unless you understand how to take into account the impedance of the elements (antennas, receivers, and junctions) and the effect of tat impedance transferred along a transmission line, you will get what look to be random results.

Yep, that's much better.

Of course, as discussed in another thread, most don't have the understanding and equipment to do any meaningful tests.

Yep, the best answer is to tell them not to try anything.

You combine this all together and the "experimenter" is more likely to develop myths and superstitions than any type of education.

That is absolutely correct. It is _much_ better to place blind faith in theoretical doctrine and never try anything outside the box.

Excuse me. I see there is someone having fun down the street and I need to go break it up before a trend develops.

 

[fmulder13]

I should have mentioned earlier this is going to be a mobile installation. Does that make a difference? The cables for both antennas are both going to be 17ft.

 

[N_Jay]

I should have mentioned earlier this is going to be a mobile installation. Does that make a difference? The cables for both antennas are both going to be 17ft.

Point made.

Go tell him to have fun, Don!:roll:

 

[N_Jay]

. . . That is absolutely correct. It is _much_ better to place blind faith in theoretical doctrine and never try anything outside the box.

I think waves and fields has progressed a bit past "blind faith in theoretical doctrine", but you just may invent a new antenna that all the people who have studied in the field have missed. So go have fun.

Some study the weather, I prefer rain dances!

 

[Don_Burke]

I should have mentioned earlier this is going to be a mobile installation. Does that make a difference? The cables for both antennas are both going to be 17ft.

I would not expect great things from that since there are many more variables running around. Mobile antennas are compromises anyway and I expect you will find a single multiband antenna will outperform that setup.

 

[Don_Burke]

I think waves and fields has progressed a bit past "blind faith in theoretical doctrine".

They thought that back in the early 1900s when all frequencies below 200 meters were deemed useless.

 

[fmulder13]

Thank you, Don. I appreciate it.