high swr with co-phased 102" steel whips

[Moleculo]

I just now took the time to think about the math after I read 'Doc's last response & I gave you bad advise also.

I said:

Yeah that's the standard co phase harness that you can buy from a lot of radio shops/truck stops. The 18' is still the wrong length to make this work properly. The coax is going into a 50 Ohm connector, plugged into a 50ohm output of the radio. That's like having about a 3 inch piece of 50 ohm coax attached.

When I was commenting on this before, I was also assuming you had some 50 ohm coax going from the radio to the cophase harness, but then when you described it in more detail, I didn't take the time to figure it out and just posted up. Oh well, Murphy's Law, I guess.

The 18' harness should be close enough if it's plugged directly into the radio / amp. If you're not getting a good match, you need to mess around with the length of the antennas as others have suggested. Here's the math 'Doc was describing in brief: The 1/4 wavelength of 27.185Mhz (channel 19) is about 108 inches. To determine the 1/4 wavelength of coax, you have to include the velocity factor of the coax in the equation, which is about 0.66 for that 75ohm coax. so 108 inches x0.66 = 71 inches which is real close to 6 feet. That means the 1/4 wavelength of this coax for the frequency in use is 6 feet. Remember where I said you needed to use odd 1/4 wave multiples for the 75ohm coax? Well the next 1/4 wave multiple is 3/4 wave, so 3x6 feet = 18 feet on each leg.

Bottom line...sorry about the bad advice earlier.

You're going to have to mess with the length of the antennas to get the SWR to where it should be. You can fine tune the length of the coax if you want, but you're close enough.

 

[ron103067]

701 try putting a 6 inch shaft on each antenna and try that. The fiberglass antennas are not as good IMO as the steel whip and really wouldn't change your readings much plus won't take as much abuse I.E. hitting low flying limbs Try the 6 inch spacer if it don't work only make one antenna live and the other for show then you always have a back up. just a thought

 

[ButtFuzz]

So let me recap without doing my homework...

.... Is the following statement correct?:


Linear or No Linear, you've got 1:2.0 vSWR or LESS across your desired transmit band and you are running twin 102' SS Whips (which have no matching network or coil)... is this a correct statement?

Run it and forget about it. Your "loss" is about 11% at 1:2, and your linear-o-matic is making up for that.

Unless this "problem" is causing super-lousy receive, you are fighting a problem that doesn't exist.

Fuzz

 

[W5LZ]

ButtFuzz,
I agree with that to a point. But there's a problem with the original assumption, the "1.2 across the band". That's just not something you should expect. Close maybe, but not very likely without a LOT of luck.
One of the problems with phasing using coax as a 'Q' section is the coax's ability to handle the voltage/current that it has to handle when making impedance changes. There just aren't that many, if any, coax cable's made that can handle that with ease, depending on the amount of power fed to the antenna system. That's a variable 'limit', not the same for every situation. Using coax as a 'Q' section is probably the easiest method, but since when was the 'easiest' way always the 'best' way?
I know that isn't what you were getting at, and I do agree with that aspect of it. There's just never -one- aspect to consider though. Just something to think about.
- 'Doc

 

[kd-5-bgt]

One thing that I done to tune duel antennas was to mount the antennas where I wanted.Made sure that they had a bit of space between the cab of the truck and the mounts(about 12 inches from cab) ,made sure that the coax would handle the power of any kicker that I might put in line.

To tune the antennas I took a straight(uncophased) run of coax of the same type(in my case mini 8) and run the coax to one of the antennas(without the kicker inline) and tuned for low swr then moved to the other antenna and tuned as needed before hooking up the cophased coax,then I fine tuned as needed before hooking the kicker back inline.With the kicker off I again would check swr to make sure there was no problems with the kicker just being inline.

If all checked out,then apply power to kicker at low power and then move up the power levels to full power rechecking the swr at each power setting......

I dont know what type of coax you were using for your set up ...... but just make sure that whatever type of coax you are running will take the power you will use.I personally use nothing less than mini 8 ...no rg 58,59....that way I know that I am good for power levels up to 700 watts at least

 

[ButtFuzz]

and don't forget...

about all those other variables... again... I didn't do my homework...but mentioning a few...

With a straight 102" whip, no matching network... hard to get less than 1.5 to 1 SWR with a single at any given frequency due to impedence of about 72 ohms at feed point. Add to that problem twin 102" whips, and a 75 ohm coax in the harness trying to make a decent compromise for two antennas that are typically expected to be in the 50-some ohm range.

It is, indeed, a lot to thing about, and some of it, I am sure, is not within the realm of me solving.

That is why I said "just run it".

Anyways, I did see one suggestion for a single whip on the top of the vehicle. I did that once and it was worldwide. Unfortunately, it was also neighborhood tall, and I was whacking into every traffic signal and underpass I encountered!!!

You guys have at it, but I gave up long ago trying to fight to get that last little nth of SWR...

Take care,
73
ButtFuzz
KG4ZPN

 

[W5LZ]

"...gave up long ago trying to fight to get that last little nth of SWR..."

Best advice I've seen in quite a while!
- 'Doc

 

[mackmobile43]

just set it up out of phase like the keydown rigs do and you'll be fine.

 

[WR0220]

...the co-phase harness is rg-59, i made sure and checked that when i bought it...

...as far as the harness is concerned, it is a pilot truck stop harness. the way it is built is that there is no "t" connection anywhere. it is two seperate pieces. one piece comes from each antenna, and they do not meet until the connector that goes to the radio, that is the only spot that the two pieces of coax join...

Isn't RG59 70/75 ohm coax? Wouldn't he need to run a 50 ohm piece of coax between the connector where the two pieces of RG59 join and the radio in order to offer an impedance the radio would like?

 

[Robb]

Isn't RG59 70/75 ohm coax? Wouldn't he need to run a 50 ohm piece of coax between the connector where the two pieces of RG59 join and the radio in order to offer an impedance the radio would like?

You see? This is what I said earlier on this thread. I thought that was the 'textbook answer'.
Doesn't that info apply to this situation equally - as well?!?

 

[Moleculo]

You see? This is what I said earlier on this thread. I thought that was the 'textbook answer'.
Doesn't that info apply to this situation equally - as well?!?

No, the output of the Radio is 50ohms. The length of the 50ohm section doesn't matter; the lengths of the 75ohm sections do

 

[hotrod]

id just use the single 102 and for get it.the duals wont give ya much if
any difference unless thier around 36ft apart.just my 2 cents worth

 

[kamikaze701st]

well guys, i figured it out. i know have my two 102" steel whips on VERY sturdy mounts. but, as i kept adding heighth to the whip, first with a 6" extension. that dropped my swr from about a 2.2 or so to down to about 1.6. then, i went to the local pilot and bought two 4" heavy duty springs made for up to 108" whips. i put those on top of the shafts, and the 102's on top of the springs and presto, my swr is now 1.3 and pretty stable. the whips kick back at a 45 degree angle at 65 on the freeway and looks pretty cool from what im told from others in our club. i went to a shootout today in fife, washington and took first place in the single final radio (bearfoot) class, which is pretty good i guess considering it was my first shootout. anyways, thanks for all of the advice everyone. i definitely know where to go when i have any issues.

"Kamikaze"

 

[CrazyCountry-NYC]

id just use the single 102 and for get it.the duals wont give ya much if
any difference unless thier around 36ft apart.just my 2 cents worth

HOT ROD..........36' apart? tell me more.....

 

[The DB]

Hotrod was wrong, six and a half years ago anyway. The optimal distance between antennas for such a cophased setup is between 18 and 22 feet. You will actually loose gain going beyond those lengths.

If you go beyond the cophased setup, it is possible that with two antennas between 4.5 and 9 feet apart will actually outperform this in one direction with phasing. It is a bit more complicated to set it up though...

You might just get response out of him though, he is still an active member here...


The DB