108" whip swr

[camaro1]

108" whip mounted about 20' up on the peak of my house, 4 copper wires used for a ground plane about 9' long each, 2 angle down and 2 are at 90* to the antenna. 50 ft coax to the radio

swr with the internal radio meter shows 1.6-1.7 but on my external meter with a 2' jumper hooked in shows 1.2-1.3 and now the radio also shows 1.2-1.3

took the meter out and kept the 2' jumper in and now the radio internal digital meter also agrees with the external meter at 1.2-1.3

whats the correct reading?

 

[W5LZ]

The correct reading is 'both'. They weren't the same, b ut neither was the overall length of the feed line. If you remove that 2 foot jumper I'd bet the SWR would change again.
You can also draw another 'correct' conclusion from that. Your antenna isn't as close to "tuned" as may think it is. That's demonstrated by the change in SWR because of the change in feed line length. If your antenna was close to 50 ohms with no reactance present (the definition of a correctly tuned -resonant- antenna), a change in feed line length would make no difference in SWR.
[And because the change in SWR was so slight, your antenna system IS close to being 'correct'. Why'z that? Because of -where- you are doing the SWR measuring, at the transmitter end of the feed line which makes the feed line part of the 'equation'.]
- 'Doc

(The first three sentences answer the question asked. The rest of all that just says why.)

 

[camaro1]

yes it did change on the radio meter removing the jumper, with the 2' jumper the radio shows 1.2 on ch1 and 40 and with the jumper removed it now is showing 1.3 on ch1 and 1.5 on ch40

 

[The DB]

As doc said:

Because of -where- you are doing the SWR measuring, at the transmitter end of the feed line which makes the feed line part of the 'equation'.

The only way to know what the antenna's SWR is is to measure the SWR at the antenna itself, wherever it happens to be. If you don't the coax in between the antenna and the SWR meter are now affect your readings. While there are ways to minimize the effects of coax, it will always have an effect.

You mentioned 50' of coax, what kind is it?


The DB

 

[camaro1]

it is rg58,, so i actually should take the meter up to the antenna and have someone else key the radio up for me?

itts a 102" whip and i had a threaded spacer to take it to about 108", because lots of post say you need the spring to make up the diff from 102 to 108"

i just took the spacer out and now its just a 102" whip,, now my radio swr readings match the external meter it now says ch1 = 1.1, ch20=1.1 and ch40=1.2


just took reading at the antenna and its about .1 higher than the readings at the radio, ch1=1.2 ch20=1.2 ch40=1.3

now this brings another question,, what is correct 102" or 108",, many people say you need the spacer or spring with the 102 whips??

 

[Brake Weight]

I get better meter readings in my truck without the spring.

 

[The DB]

246 / 27.185Mhz = 9.049... feet

246 is from the ARRL antenna book for finding the length of quarter wave antennas in feet
27.185Mhz is the middle frequency in the CB band

9.049... feet is just over 108 inches.

246 / 29.0Mhz = 8.482... feet

29.0Mhz is close to the middle of the 10 meter ham band

8.482... feet is just under 102 inches

As you can see the length of the 102" whip is not in the CB band, which is why a spring/riser i said to be needed.

These calculation did not calculate width as the width of a 102" whip is not significant enough to matter.

I get better meter readings in my truck without the spring.

Readings with just an SWR meter or an Antenna Analyzer?


The DB

 

[Marconi]

Cmaro1, my experience and my modeling tells me if the 102" whip has a good ground plane attached at the feed point, then you can see a near perfect match similar to what you tell us about your antenna.

When the ground plane is not so well applied, like a mobile, then you may need to make the antenna longer.

A 1/4 wave whip antenna is very dependent on the ground plane, and this might help explain why so many guys have trouble mounting a 102" whip on their mobile only to see a high SWR.

 

[555]

used antron 99 shipped to your door for 25.00 and be done with this

 

[camaro1]

thanks for the help, i think im good to go. i just couldnt understand the 102" vs 108" calculated length. the swr's are good at the antenna and the radio, dont have any type of analyzer so thats about as good as im going to get it, maybe upgrade the coax sometime but it all seems to be working great so far

 

[Brake Weight]

Readings with just an SWR meter or an Antenna Analyzer?


The DB

Just an SWR meter. Ranges from a 1.2-1 up to a 1.3-1 from one end of 11m to the other.

 

[The DB]

Just an SWR meter. Ranges from a 1.2-1 up to a 1.3-1 from one end of 11m to the other.

I was thinking that was likely the case.


The DB

 

[W5LZ]

Use '234' instead of '246' and you'll get closer to the actual resonant length for a typical 1/4 wave antenna. Read the rest of that chapter in the Hand Book, you'll find where that '234' comes from.
By those radials slopping down, you will increase the input impedance of a typical 1/4 wave antenna to some extent, and that's evident from your readings. Using horizontal radials the input impedance would be closer to 25 ohms than 50 ohms so the resulting SWR would indicate that.
From what you've described, you've got it about as 'close' as I think you'll get. I'd leave it as-is.
- 'Doc

 

[Greatvalue1]

I don't know if this is the most accurate way of "Hillbilly" tuning, but I used a 1/2 wavelength of coax (1/2 wavelength x velocity factor) 14'-10" of RG8x.

My calculations may be sketchy, but I've heard that an odd multiple of a half wavelength of coax will give a decent reading at the meter.

Please let me know if this is inaccurate.

 

[BOOTY MONSTER]

a E1/2WL or any multiple of it will help take the coax out of the equation FWIU .

Coax information

" ........ WHAT LENGTH?
As mentioned the best length is the shortest, in this way the loss due to the cable is the lowest.
There can be one advantage by extending the length of the cable:.
A HALF WAVE coax cable or ANY multiple (1, 1½, 2 wavelength) length of this will show you: "exactly the same input resistance at both ends of cable". In normal words:
You can measure the exact SWR from the antenna with these lengths of coax cable.

If your antenna is truly 1:1 it doesn't matter, each SWR measurement anywhere on the line and you will find 1:1.
Only when your SWR isn't 1:1 but higher your measurements will change according to length, this is when it can come in handy to keep the line a half wave long (or any multiple). ...... "