Modified Vector 4000

[bob85]

That's the old T-piece trick Eddie

stubs are now connected in parallel with the 50ohm dummyload,

in this case the open ended 1/2wave repeats an open circuit with no reactance where its an exact electrical 1/2wave,

the shorted 1/4wave inverts to an open circuit with no reactance where its an exact electrical 1/4wave,

leaving just the 50ohm load seen by the analyzer

 

[Alexis Mercado]

At last I could tune the Vector. I had to shorten L3 to 23 5/8 inches and L2 adjusted to 24 3/4 inches.

Here are the numbers from the MFJ analyzer measured with a 1 foot of LMR-240 jumper on 6 feet mast on top of my roof.

SWR———-Frequency———-R———X

2.0 ———— 27,805—————42——-3

1.5————-27, 504—————52——-21

1.0————-27,085—————-50——-0

1.5———- 26,705—————-34——-10

2.0————-26, 500—————25——10

Bandwidth: 1,305 KHz

I replace the short jumper for a 14 feet RG8X jumper. It gave me the same readings. Like someone said on previous post ( I don’t recall ), as long as the antenna shows 50 ohms at feed point, coax length doesn’t matter.

But ( there is always a BUT ) when I elevated the mast to 12 feet, SWR went up to 1.2 , R= 35 and X = 9 on 27,085.

I lowered it back to 6 feet and SWR went down back to 1.0.

I wanted to see how it behaved with the coax where everything is connected, I hooked it, to see readings on SWR meter.

Here they are, at 6’ feet high and 12’ feet high

SWR—Freq. 6’——Freq. 12’—-Freq. 16’

2.0— 28,115——— 28,225———28,400

1.5—-27,690———27,665———-28,050

1.0—27,105—(1.2 @ 27,035)—( 1.35@27,725)

1.5—26,680——— 26,595———-26,935

2.0—26,435——— 26,300———-26,230

BW at 6’ high = 1,680 KHz.
BW at 12’ high= 1,925 KHz.
BW at 16’ high = 2,170 KHz

As you can see on the data above, when I elevate from 6’ to 12’, SWR tend to raise a little, from 1.0 to 1.2 ( on both, SWR meter and analyzer) . Antenna tend to get more bandwidth.

The more I elevate it, the higher the SWR and more broad banded it gets. I elevated it once at 16 feet high and see what happens, more broadband, lowest SWR shifted up in frequency .

Why does this happen ? Perfect match with analyzer and SWR meter at 6 feet high but things change when elevation is changed.

Here are some pics for you to see. Sorry about the orientation. I don’t know how rotate pictures as they should be.

 

[bob85]

I don't see a choke at the feed-point Alexis ?

Quick test to try,
Disconnect the ground strap & see if that changes vswr with the mast at the different heights you have tested in the chart above,

If it does you have common mode on the mast & ground strap which will change with its electrical length & been grounded or not grounded at the ground rod,

Also as you raise it up you have less coax laying on the roof,
if you have common mode on the coax that could also change the tuning.

If its not that it could be that your roof has rebar in it & that's effecting your tuning.

Distributed capacitance to ground or other conductor under the antenna will effect tuning,
more height less distributed capacitance.

 

[Alexis Mercado]

I don't see a choke at the feed-point Alexis ?

Quick test to try,
Disconnect the ground strap & see if that changes vswr with the mast at the different heights you have tested in the chart above,

If it does you have common mode on the mast & ground strap which will change with its electrical length & been grounded or not grounded at the ground rod,

Also as you raise it up you have less coax laying on the roof,
if you have common mode on the coax that could also change the tuning.

If its not that it could be that your roof has rebar in it & that's effecting your tuning.

Distributed capacitance to ground or other conductor under the antenna will effect tuning,
more height less distributed capacitance.

Thanks Bob. I forgot to mention, SWR does not change if I disconnect the ground strap, not even a little.

And of course, roof has rebar all over the place. It is a concrete roof reinforced with rebars.

I think I tested with the line isolator, with no difference but, I’ll try tomorrow again and will let you know.

 

[bob85]

Im leaning towards the roof been the probable cause,
try the isolator at the antenna & see if theres any change in tuning,

good luck

 

[Alexis Mercado]

I have a vague recollection using this method. I think I could test the VF of a piece of coax, and maybe even test my tuned 1/2 wave jumper to see if it was spot-on the frequency I wanted.

Not sure how successful I was at clipping the coax length in the process of tying to construct a tuned 1/2 wave or multiples however.

I also seem to remember this idea was not always as easy as the words made it seem.

Here is an old video on a similar idea

Thanks Marconi for this video. I followed this video and could calculate 1/4 wave resonant jumper. I set the analyzer in Impedance R & X on 27,205 KHz.
( Channel 20 ). In order to use this method, one must cut the coax longer like Dr_DX said, to cut coax 20% longer ( I recommend to use formula first and then add the 20% to play with it) and cut little by little in order to find resonance.

I think it is more convenient to calculate a 1/4 wave length ( less hassle and cutting valuable coax ) and then multiply it by the odd or even multiple you will need.

When R = 0 and X = 0 then I began to cut little by little until X disappeared like in the video.

Resonant 1/4 wave jumper length at 27,205KHz calculated by my MFJ-269 C analyzer is 7.25’ = 7’,3” long. Using formula give me 7.5926 = 7’,7”. If I have around 50 feet from antenna to station, it would be an important difference to affect resonance in the coax cable.

If I am going to make a jumper from antenna to station, ( 50 feet distance ) should cut the cable by odd or even length ? ( I plan to use 7.25’ given by the analyzer).

Is it right my thinking of multiplying by X multiples of the result given by the analyzer?

For sure I know coax length is not important as long as antenna is resonant on the frequency you want it to be, but sometimes cutting coax for resonance as well, can resolve some SWR issues like I am having right now with my Vector 4000.

By the way, Bob85 said to use choke ( I used a line isolator ) but adding a line isolator got things worse. I could not get SWR lower than 1.5 and bandwidth was higher. From a SWR of 1.3 and a BW of 2,070 KHz @ 16 feet high above roof, by adding the line isolator,SWR went up to 1.5 BW went up to 2,470 KHz @ 16 feet high above the roof.

 

[bob85]

Alexis
If adding the line isolator is upsetting your vswr its an indication that you have cmc on the braid,
if there was no cmc, a good isolator should not change vswr notably.

 

[Alexis Mercado]

Hi Bob. I used two type of line isolators, a Radio Works T-4 and a CMC-330-1K.

Using good coaxial. As well. Antenna grounded. Then, What else can I do ?

 

[bob85]

Alexis,

you will have to alter the length of the antenna to get it to tune if you can't tune it by moving the gamma strap & gamma rod,
different heights above your roof will likely need some adjustment of all 3 adjustments on sirio's tuning chart,

the 1/4wave jumper you made will invert the impedance of the antennas feedpoint whenever the antenna is not 50ohm,

a 1/2wave jumper will not invert the impedance on the frequency the jumper is tuned for even when the antenna is not 50ohm,


whatever length jumper you use the coax will shift where you see resonance x=0,
unless you are amazingly lucky & the antenna is resonant at the same frequency the coax is resonant.


Good luck

 

[Dr_DX]

Resonant 1/4 wave jumper length at 27,205KHz calculated by my MFJ-269 C analyzer is 7.25’ = 7’,3” long. Using formula give me 7.5926 = 7’,7”. If I have around 50 feet from antenna to station, it would be an important difference to affect resonance in the coax cable.

If I am going to make a jumper from antenna to station, ( 50 feet distance ) should cut the cable by odd or even length ? ( I plan to use 7.25’ given by the analyzer).

Is it right my thinking of multiplying by X multiples of the result given by the analyzer?

For sure I know coax length is not important as long as antenna is resonant on the frequency you want it to be, but sometimes cutting coax for resonance as well, can resolve some SWR issues like I am having right now with my Vector 4000.

By the way, Bob85 said to use choke ( I used a line isolator ) but adding a line isolator got things worse. I could not get SWR lower than 1.5 and bandwidth was higher. From a SWR of 1.3 and a BW of 2,070 KHz @ 16 feet high above roof, by adding the line isolator,SWR went up to 1.5 BW went up to 2,470 KHz @ 16 feet high above the roof.

Alexis,

Let's use the 7.25' determined by your analyzer for your specific RG-8x. Remember that is a 1/4 wavelength. So we would double it for a 1/2 wavelength - 14' 6". You want to use multiples of a 1/2 wavelength to feed your antenna. So 3 x 14.5' = 43' 6" - probably too short. 4 x 14.5' = 58' which will then reach. Use a 58' piece of coax and tune for lowest SWR on channel 20 (27.205). Make sure antenna has a line isolator and is at the operating height.

Like Bob said, it sounds like you have CMC on your coax, so you definitely need to use a line isolator. You will need to adjust the length of the antenna to get the SWR down - you can't only adjust the Gamma.

 

[Alexis Mercado]

Yesterday I could finally use the analyzer with the 7.25’ feet long of LMR-400.

Alexis,

you will have to alter the length of the antenna to get it to tune if you can't tune it by moving the gamma strap & gamma rod,
different heights above your roof will likely need some adjustment of all 3 adjustments on sirio's tuning chart,

the 1/4wave jumper you made will invert the impedance of the antennas feedpoint whenever the antenna is not 50ohm,

a 1/2wave jumper will not invert the impedance on the frequency the jumper is tuned for even when the antenna is not 50ohm,


whatever length jumper you use the coax will shift where you see resonance x=0,
unless you are amazingly lucky & the antenna is resonant at the same frequency the coax is resonant.


Good luck

Hi Bob. What do you mean that the 1/4 wavelength jumper I made ( by the way it’ s LMR-400 ) will invert impedance , while the 1/2 wavelength will not ?


Dr._DX

These last days, I was playing with the measurements of the Vector 4000 and could drop it to 1.0 on channel 8.

L2 ( Gamma ) is set to 24 3/4” and L3, which is the top section is set to 28 3/4 “ With the analyzer close to the feed point , at 6 feet high, I had a nice dip on the SWR of 1.0 with an R = 50 and X = 0.


Tomorrow I’ll give you more readings using different coax lengths, etc.

 

[bob85]

Alexis
a 1/4wave of coax or odd multiple 1/4waves transforms impedance whenever the load is not perfectly matched to the coax,
a short circuit at the far end looks like an open circuit at the analyzer,
an open circuit at far end looks like a short circuit at the analyzer,

if your antenna was 25ohm you would see 2:1 swr but impedance would be inverted to 100ohm at the analyzer,
a 100ohm load will be 2:1 swr & invert to 25ohm at the analyzer,

you can test this by putting resistors or 2 dummyloads on a t-piece at the end of your 1/4wave jumper,

if you do the same with a tuned 1/2wave of coax, you will see that the 1/2wave does not invert impedance when its not perfectly matched,

a 1/2wave reflects the load impedance at its input end,

both lengths will shift where you see resonance at the analyser`,

tune for minimum swr since minimum swr occurs when the load has least reactance.

i would use your best line isolator at the feedpoint & include it as part of your tuned 1/2wave jumper when you tune the jumper.

 

[Alexis Mercado]

It was a good thing to have it tuned on channel 8 because I know that as soon as I raise the mast up, SWR and resonance frequency tend to raise as a whole. Check this out.

I hooked the analyzer with the 1/4 wave , 14’ feet tuned jumper I made out of LMR-400, 6 feet above roof.

SWR——-Frequency ———R————X
2.0————-27,755————-25———-8
1.5————-27,445————-33———-7
1.0————-27,055————-50———-0
1.5————-26,705————-69———16
2.0————-26,485————-72———37

BW= 1,270 KHz


Meter readings at station , antenna @ 16 height above roof

SWR Freq——-Freq + Isolator—Coax close to mast
2.0——28,055———28,265——————-28,165
1.5——27,705———27,865——————-27,775
1.15—-27,235——( 1.3) 27,445———-(1.3) 27,225
1.5——26,705————26,585———————26,585
2.0—-26,265————26,105———————26,145

BW—1,790 KHz——2,160 kHz————-2,020KHz



Why do bandwidth tend to raise SWR and resonant frequency a little?

Bu the way Bob, I did have CMC issues. Before attaching the Line Isolator, I attached the coax cable to the mast and things changed, SWR went up a little and bandwidth widened.

For some reason I am going to check today, it’s the fact that for some reason I hear some local stations with RX and the also copy me about 2-3 Db lower than before.I really don’t have any idea why this happens .

 

[bob85]

2-3db is a big change in signal Alexis,
what height & coax length are causing the change in signals ?

im leaning towards it been your roof that's effecting the tuning,
tuning 6ft above all that rebar in the roof then raising the antenna up away from it,

i had the same thing happen when tuning 2ft or less from ground then raising the vector up to 12ft & 18ft at my old location.

 

[Alexis Mercado]

Bob, right now it is 16 feet above roof. I’ ll take a picture of the isolator installed. I am thinking about insertion loss caused by both a 3” jumper connecting isolator to antenna and the isolator itself. I, ll take it out and I’ll try again.