Dusterbd13
Dusterbd13 MegaDork
9/5/18 4:54 p.m.

So, the factory antenna in the e39 ain't all that great. Its mounted in the glass.

I had planned to bypass it and use a metra amplified roof antenna. 

But then i had a thought. Why not run BOTH!

i can get an antenna connector y piece to do so.

But, would it make it better for digital fm reception?

bentwrench
bentwrench SuperDork
9/5/18 5:31 p.m.

Better to have a switch between the two, the problem with FM in town is not usually signal strength but multi-pathing.

Dusterbd13
Dusterbd13 MegaDork
9/5/18 6:38 p.m.

The problem is that the stations i like are a minimum of 50 miles from here.  Im in the stcks.

flatlander937
flatlander937 HalfDork
9/5/18 7:28 p.m.

Based on the reading I did regarding CB antennas years ago... two antennas means the vehicle as a ground plane for both cancels each other out and the signal gets sent primarily forwards/backwards... and not so much side to side.

 

A quick google found this which may be of interest to you:

https://www.eng-tips.com/viewthread.cfm?qid=285231

flatlander937
flatlander937 HalfDork
9/5/18 7:29 p.m.



jj
jj HalfDork
9/5/18 7:48 p.m.

I’ve not seen dual antennas used commercially for better reception.  The only thing I’ve seen is getting the antenna higher and having a more sensitive receiver.  Or if it is non-mobile using a directional antenna. 

Dusterbd13
Dusterbd13 MegaDork
9/6/18 6:59 a.m.

Thanks guys. I figured there was a reason its not common to see multiple fm antennas. 

The roof mount will be as high as we can get, and my kenwood has sime good fm sensitivity specs.

stuart in mn
stuart in mn UltimaDork
9/6/18 9:11 a.m.

When I think of dual antennas I think of this.  smiley

 

Dusterbd13
Dusterbd13 MegaDork
9/6/18 9:17 a.m.

In reply to stuart in mn :

Pretty much what gave me the idea

Aspen
Aspen Reader
9/6/18 11:11 a.m.

Best and cheap would be a single stick type antenna about 30" long for FM, mounted vertically on the roof, but it will look awful.  Maybe a power retractable type on the rear fender?  Vertical is better than semi-horizontal. Hard to beat the standard antenna used for decades.  For digital radio a shorter antenna may actually be better, it depends on the frequency you are trying to tune in.  There are charts you can look up.

Amplified antennas only work if they have a good signal to amplify.  Lousy signal amplified is just more lousy signal.

50 miles is far for FM in car, a house not so bad because you can use a big Yagi mounted high on a tower and a good DX tuner made in the 70s.  Hard to replicate that in a car.

GTXVette
GTXVette SuperDork
9/6/18 11:31 a.m.

Dual curb feelers are next. or cone feelers for the Miata.

jj
jj HalfDork
9/6/18 12:02 p.m.
Aspen said:

Best and cheap would be a single stick type antenna about 30" long for FM, mounted vertically on the roof, but it will look awful. 

I agree, a 30" whip would work best.  If you have a friend in the communications biz, they can trim the whip for best vswr after it's mounted to the car since the car will have some effect on the reception.

N Sperlo
N Sperlo MegaDork
9/6/18 12:22 p.m.
flatlander937 said:



This is more for transmitting than receiving. Yes, there is an effect, but minimal. I'd suggest running one antenna. The higher, the better. Planning on putting my 102" on the truck soon. 

FM transmitters are vertical, therefore, vertical should be the best position for your antenna. 

Toebra
Toebra HalfDork
9/6/18 2:37 p.m.
stuart in mn said:

When I think of dual antennas I think of this.  smiley

 

One of those antennas is ersatz, not hooked up to anything.

pres589
pres589 PowerDork
9/6/18 4:13 p.m.

How sub-optimal would a ~30" whip antenna do on the FM side of things if the antenna was run along the top of the windshield inside the car?  Like from p-clamps with good adhesive foam tape or similar.  I know it's laying in a horizontal position and not vertical but is that major?

I've toyed with this idea in the past to improve reception in my car where I currently have a Metra "Hide Away" electrically small antenna.  It doesn't work that well.

Aspen
Aspen Reader
9/6/18 4:35 p.m.

In reply to pres589 :

Very...inside is the killer,  it has to be outside or the metal of the car is shielding the antenna.  Horizontal is also poor so together you will get zilch.

N Sperlo
N Sperlo MegaDork
9/6/18 4:50 p.m.
Aspen said:

In reply to pres589 :

Very...inside is the killer,  it has to be outside or the metal of the car is shielding the antenna.  Horizontal is also poor so together you will get zilch.

This. Your car is practically a Faraday cage. Being in the window won't do much to help that fact. 

I actually think a home made 75 ohm antenna would work... Slightly? Better? It's still in that cage, though. 

jj
jj HalfDork
9/6/18 6:08 p.m.

In reply to pres589 :

Horizontal wouldn't hurt much, but being inside will.  Radio likes line of sight, so anything inside will be less effective.

pres589
pres589 PowerDork
9/7/18 9:31 a.m.

Cell phones working inside a car body disproves this 'Faraday cage' idea.  Any RF engineers want to weigh in?

Dusterbd13
Dusterbd13 MegaDork
9/7/18 10:19 a.m.

Howabout a link for antenna length for digital fm reception for different bands? The stations that i want are 106.5 and 100.3

Both their hd2 stations are berkeleying AMAZING. 

Aspen
Aspen Reader
9/7/18 11:38 a.m.

the standard 29.5" whip will be good for those stations 'cause it is optimal for the middle of the FM band at 98.  In theory the 100.3 station should be received better, but it's distance from tower to your car and the terrain in between as well as signal power will have more effect.

Cell phones have different frequency so not as affected by the car metal as FM is,  so they work inside a car.  The antenna tech inside your phone is pretty sophisticated stuff using fractals and lots of math.  There isn't the same kind of math going on in an FM antenna mainly because the standard whip works so well, is cheap and OK to look at.  Also I recall reading that the whip antenna works with the radio waves bouncing off the flat plane of the hood/roof/deck lid to enhance reception, that's why a outside vertical mount works best for FM.

jj
jj HalfDork
9/7/18 5:54 p.m.

In reply to Dusterbd13 :

28.5 inches would be best tuned for those frequencies.  However, that would be for an antenna in isolation.  When it's attached to your car it changes things a bit.  It's hard to get it perfect in real life.

Dusterbd13
Dusterbd13 MegaDork
9/7/18 6:11 p.m.

Is it still a good number to shoot for? The amplified roof antenna i just installed has a roughly 18 inch mast. Im sure i can fit a 30 if needed before it whacks low hanging branches. 

jj
jj HalfDork
9/7/18 7:01 p.m.

Yes, it’s still a good number. You can calculate the best whip length in most cases with this formula:

The speed of light in inches per second divided by the frequency (100Mhz = 100,000,000hz) times 1/4.

 

stuart in mn
stuart in mn UltimaDork
9/7/18 8:10 p.m.
pres589 said:

Cell phones working inside a car body disproves this 'Faraday cage' idea.  Any RF engineers want to weigh in?

A car isn't a perfect Faraday cage - there are glass windows.

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