another antenna tuning question

[longhaireddwb]

Working on a new kenworth. The driver put new wilson 2000 antennas on it. Two of them as its co-phazed coax. I put my antenna anylizer on the feed point for both antennas. Where the radio plugs in.
Im getting a swr of 1.1 to 1.2 across the band. Great. I then thought id check the antennas with a single 50 ohm coax and they are reading 1.6 with a impedance of 122 each side give or take.

Why would this be?

I plan on going with the first adjustment as it reads great from the feed point but why so much differance? Sould an antenna adjusted to phazed use be so much different from single use?
Anyone?
Learning factor for me and maybe a few others folks on this site as well.
Thanks.

 

[longhaireddwb]

I guess what is bothering me is the math.

If you have two 75 ohm coax run parallel shouldn't the impedance of the coax be 37.5 and then when you add the two 50 ohm antennas to it shouldn't it add up to 62.5 give or take.

Now if you have a single 50 ohm coax with a 50 ohm antenna shouldn't the impedance be 100?

But when you tune a single antenna on a single coax (remembering the coax has been cut to length using the velocity factor math to make it a quarter wave) the impedance comes out to around 50 when everything is properly tuned.

Please help me to understand this. As I said I found with my coax on a single antenna in the first post was impedance of 120 or so.

Maybe I'm just getting too picky?

 

[W5LZ]

"If you have two 75 ohm coax run parallel shouldn't the impedance of the coax be 37.5 and then when you add the two 50 ohm antennas to it shouldn't it add up to 62.5 give or take."
You're missing a component of that antenna system, you aren't including the input impedance of the antennas. So think of it like this...
A typical mobile antenna has an input impedance of something like 30 - 35 ohms, not 50 ohms. So, 35 ohms plus that 75 ohm coax would give you around 100 - 110 ohms. Now, if you parallel another antenna/coax you would end up with something around 50 to 55 ohms, which would work out to a good match to a 50 ohm radio, right? Nothing is ever exactly 'right' so there's no telling what you will end up with, something close to right is doing pretty good. The most common 'variable' in this sort of antenna system is the input impedance of the antennas.
That help any?
- 'Doc

 

[9C1Driver]

Working on a new kenworth. The driver put new wilson 2000 antennas on it. Two of them as its co-phazed coax. I put my antenna anylizer on the feed point for both antennas. Where the radio plugs in.
Im getting a swr of 1.1 to 1.2 across the band. Great. I then thought id check the antennas with a single 50 ohm coax and they are reading 1.6 with a impedance of 122 each side give or take.

Why would this be?

I plan on going with the first adjustment as it reads great from the feed point but why so much differance? Sould an antenna adjusted to phazed use be so much different from single use?
Anyone?
Learning factor for me and maybe a few others folks on this site as well.
Thanks.

A local guy here would put the antenna analyzer at the feedpoint. Then he would remove one antenna and put a 50 ohm dummy load in it's place. Then set the SWR on the remaining antenna. Then reverse the process and do the other side. When done with both antennas on it should be good to go.

 

[Captain Kilowatt]

Since when did adding the coax impedance to the antenna impedance equal the actual impedance of the overall system? :blink:

That's like saying that if two pieces of 75 ohm coax in parallel equals 37.5 ohms (and it does) then two pieces of 75 ohm coax in series equals 150 ohms. It doesn't. It still equals 75 ohms.

BTW in the case of using phased antennas the two 75 ohm phasing lines are NOT in parallel. They do not have the same connection points on each end. They go to separate antenna.

 

[longhaireddwb]

You've got a big point on that one Captain Kilowatt. I hadn't thought of the both antennas separate and not connected as in a set of parallel resistors.

9C1driver... I like that idea of how to tune. It does make sense. If you put a 50 ohm dummy load in place of one side and then tune the other you are getting a real reading on the one antenna.
Does anyone disagree with this tuning procedure?

Doc says: "A typical mobile antenna has an input impedance of something like 30 - 35 ohms, not 50 ohms".

I guess I never tested out just an antenna to see if it is NOT 50 ohms. But that would make the number more correct with the math you used.

BUT... Why then do you get approximately an impedance of 50 when you use one antenna and a single coax and with DOC's math it would end up around 85:?

 

[Shockwave]

The tuning method using a dummy load on one side will get you real close but it overlooks something called mutual inductance. The effect is when two resonant antennas are combined, the resonant frequency will shift. The best way to handle this is to tune them individually then combine them through the phasing harness. Note the change and direction in resonant frequency. I forgot what way it goes right now.

Nonetheless, you should be able to find a frequency close by where the match is good again if the phasing harness was cut right. If you have a good match 200 KHz. down from where they tuned individually, then retune them individually at a frequency 200 KHz. higher than the desired one. Once combined again the match should be good and even between the two.

One reason you are seeing such a high impedance on a single antenna is the lack of ground plane under the antennas. The co-phase truck mounting situation has issues with getting enough spacing between antennas while having any metal underneath them. I'd just be happy you found a decent match with them combined the way you want to use them.

The 35 ohm 1/4 wave impedance only takes place when the 1/4 wave is mounted in the center of a large horizontal ground plane. Such as the middle roof section of a van. If the antenna is off to one side of the metal under it, as you see the impedance will be much higher than even 50 ohms. This is why 1/4 wave base antennas angle the radials downwards to reach a 50 ohm match and not out for 35 ohms.

 

[longhaireddwb]

The best way to handle this is to tune them individually then combine them through the phasing harness. Note the change and direction in resonant frequency. I forgot what way it goes right now.

Nonetheless, you should be able to find a frequency close by where the match is good again if the phasing harness was cut right. If you have a good match 200 KHz. down from where they tuned individually, then retune them individually at a frequency 200 KHz. higher than the desired one. Once combined again the match should be good and even between the two.

Do you think tuning this way would be better then tuning with both antennas on the phasing coax both at the same time? By adjusting the same amount on both antennas at the same time? I'm thinking by tuning them separately you would get a more accurate reading.

I'll check into this tuning procedure to see how well it works. Unless someone make a comment about it. I like doing the best job I can do when tuning antennas or anything I do. If there is a better way even if it takes me longer I want to do it that way. Thanks for the comment Shockwave!

 

[W5LZ]

Unless a mobile antenna has a matching device, it's not going to be 50 ohms impedance, probably not even close. Part of it is in how/where the thing is mounted, yes, but there are a few other considerations that have to be taken into account.
I would suggest you using an antenna analyzer to determine input impedance, a simple SWR meter isn't very useful doing that. Not a very 'palatable' idea but it's the truth.
- 'Doc

 

[222DBFL]

Or just use a single antenna. Problem solved. Less issues IMO.