Bob, at 6 feet high, I get SWR of 1.1 on frequency 27,055 with R=48 and S=8. Is that close enough for resonance?
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Modified Vector 4000
- Thread starter bob85
- Start date
its not perfect but you won't notice a difference on air if 27.055 is the frequency you want the antenna tuned for,
is that with the analyzer at the antenna or through coax ?
when making an impedance chart like you have above with a mismatched antenna measuring through coax,
the only frequency that what you see for R is what the antenna measures is the frequency where your coax is an exact electrical 1/2wave or multiple halfwaves,
on other frequencies where you coax is not an electrical 1/2wave the coax will transform the reading to some other value depending on its electrical length at the frequency you take the measurement
unless the antenna is 50ohms or a perfect match to your coax,
then coax length does not matter, a perfect match will read the same through 10ft 15ft 20ft of coax etc,
on the other hand vswr should not change much with coax length
providing you don't have common mode on the braid vswr will read a little lower the longer the coax is due to loss in the longer coax,
if you do have common mode on the braid, vswr will likely measure different with different lengths of coax.
best thing is put the analyzer at the feed-point or use a few inches of coax to allow the connection if you can't screw the analyzer to the vectors so239 & use binoculars to read it from a reasonable distance,
move the frequency & stand back again take another reading etc,
that way you don't have any coax effects or your body close to antenna to worry about,
resonance & impedance will be what the antenna measures & you can tune for r=50 x=0 on the frequency you want it tuned for,
sounds crazy but it can be done like that if you are OCD about getting your antenna as perfect as your analyzer allows.
is that with the analyzer at the antenna or through coax ?
when making an impedance chart like you have above with a mismatched antenna measuring through coax,
the only frequency that what you see for R is what the antenna measures is the frequency where your coax is an exact electrical 1/2wave or multiple halfwaves,
on other frequencies where you coax is not an electrical 1/2wave the coax will transform the reading to some other value depending on its electrical length at the frequency you take the measurement
unless the antenna is 50ohms or a perfect match to your coax,
then coax length does not matter, a perfect match will read the same through 10ft 15ft 20ft of coax etc,
on the other hand vswr should not change much with coax length
providing you don't have common mode on the braid vswr will read a little lower the longer the coax is due to loss in the longer coax,
if you do have common mode on the braid, vswr will likely measure different with different lengths of coax.
best thing is put the analyzer at the feed-point or use a few inches of coax to allow the connection if you can't screw the analyzer to the vectors so239 & use binoculars to read it from a reasonable distance,
move the frequency & stand back again take another reading etc,
that way you don't have any coax effects or your body close to antenna to worry about,
resonance & impedance will be what the antenna measures & you can tune for r=50 x=0 on the frequency you want it tuned for,
sounds crazy but it can be done like that if you are OCD about getting your antenna as perfect as your analyzer allows.
Alexis, I added a 9' foot long horizontal beam element 11' feet away and at the same height as the radial loop on my NV4K model.
This idea is similar to what your noted above, albeit is not a Mosley Yagi. I wanted to see if this horizontal metal element would have any effect on the pattern and match for the Vector...even thought there should be about <>20 db polarity difference in the radiated effects between these elements.
The model in the PDF file below shows the element hanging in Free Space about 10' feet away form the Vector loop. There is no noticeable difference to be noted in the Vector pattern, they overlay exactly as noted in the overlay image. There is only a very slight difference to be noted in the matching results in the 2 source data reports for these two models.
What did you do about this problem when you had your SP500 with long ground plane radials?
If you're seeing something going on with the beam as you rotate it, it has been my experience...that could also notice that happening a little with nothing around the beam installation, and just rotating the beam. There may be a little more difference on the beam than you see on the Vector however. I don't think that is your problem with this Vector, but that is just a guess.
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Alexis,
You said earlier that you wanted to tune the antenna for ch. 20 so try this, if you have some extra RG-8X @ 82%VF. Cut a piece 59' 3.5", put connectors on both ends, connect one end to the antenna feedpoint or Line Isolator if you are going to use it, and the other end to a SWR meter (meter, analyzer, etc.). Tune for lowest SWR on ch20 (27.205). This will give you a 2 wavelength @ 27.205 MHz piece of RG-8X coax, and it bypasses the antenna switch. Make sure the antenna is mounted where it is going to be used when tuning it. You always want to "final" tune your antenna with it up and in it's "operating" position/location.
I would highly recommend using a dedicated single piece of coax to the Vector if possible and if possible, use better coax.
With a 2 wavelength piece of of RG-8x (@ 27.205) you are going to lose 19% of your power going out and 19% of your incoming signal. With an antenna switch in between, even more due to insertion loss. With a 61' 5 9/16" piece of LMR400 (2 wavelengths @ 27.205MHz), you will lose 9% of your power going out and 9% of your incoming signal.
Imitation LMR400 can be pretty cheap and it will use a standard PL-259.
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Alexis, I agree with DR_Dx calculations. I don't know how you earlier got 14' feet however.
I get 14.8' feet for this tuned 1/2 wavelength coax cut for 27.205 Mhz.
I got 14.1 feet because I have a Tram RG-8X which states it has a 0.78 velocity factor. .1 is 1.2”.
Some post before I said it was .82 but it is .78.
I really have 14’ and 2 inches from tip to tip of each PL-259.
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I’ll do as you say. I will try the LMR-400 but it’s from DX Engineering, which has a velocity factor of 84%, which gives me 15.1851’ x 4 = 60.74’ = 60’ + 9” .
I would like to know how this cables from DX Engineering compare to the real deal, the Times Microwave LMR-400.
Please, correct me if I am wrong.
I used this page to calculate length :
https://www.qsl.net/w4sat/velfact.htm
Sorry, but my numbers are going to be off then. I thought you said your RG-8x was 82%vf, so yeah, if it is 78%, then the length will be shorter.
I believe Times Microwave LMR400 has a 85% VF. I know the imitation stuff I use is also 85%. So, again, if it is 84%, the lengths will be a little shorter. But remember, these are just factory specs. Your analyzer will be able to give you a much more accurate VF for that particular piece/brand of coax. Just use a measured piece ~20% longer than what you will need (~72'). Get an accurate determination of the VF, plug that number into the equation, determine the 2 wavelength distance, cut the coax to length.
I use the calculator found at the bottom of the page here:
https://ac6la.com/tldetails1.html
TL Details. You can DL it and run the .exe on your PC. It allows you to pick common coax types but also change the numbers/settings. You can even create a custom coax, if yours isn't listed.
I use TMS LMR240 & LMR400 exclusively for 2 reasons.
1. the loss at these frequencies (27.185) is only .64 dB per 100 feet. I appreciate the advantages of using virtually lossless lines. Line attenuation is the killer, not VSWR.
2. where the rf shielding attenuation for conventional coaxial feedlines with a single shield conductor seldom exceeds better than > 40 dB, LMR240 and LMR400 are manufactured with a dual shield which provides increased levels of rf shielding attenuation to better than > 90 dB.. That eliminates a lot of "unwanted" coupling.
1. the loss at these frequencies (27.185) is only .64 dB per 100 feet. I appreciate the advantages of using virtually lossless lines. Line attenuation is the killer, not VSWR.
2. where the rf shielding attenuation for conventional coaxial feedlines with a single shield conductor seldom exceeds better than > 40 dB, LMR240 and LMR400 are manufactured with a dual shield which provides increased levels of rf shielding attenuation to better than > 90 dB.. That eliminates a lot of "unwanted" coupling.
Having an analyzer for tuning coax & stubs eliminates all the variables like
makers tolerances, using the wrong chart & misprinted charts, fake coax, precision guesswork etc,
a good vswr meter is all you need if you don't use your analyzers capabilities
makers tolerances, using the wrong chart & misprinted charts, fake coax, precision guesswork etc,
a good vswr meter is all you need if you don't use your analyzers capabilities
Eddie,
tuning stubs in the case of the mfj analyzer you can read page 34 of the user manual,
or use a 2 port vector network analyzer & follow the instructions for the instrument.
you can also use a quality T-piece an accurate 50ohm load & good vswr meter,
the old T-piece trick people used before inexpensive analyzers were available.
tuning stubs in the case of the mfj analyzer you can read page 34 of the user manual,
or use a 2 port vector network analyzer & follow the instructions for the instrument.
you can also use a quality T-piece an accurate 50ohm load & good vswr meter,
the old T-piece trick people used before inexpensive analyzers were available.
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
i don't agree with his ideas on swr Eddie, sounds like cmc on the coax braid to me,
He's tuning a 1/4wave coax jumper for no reactance, the near infinite open circuit impedance at the far end is inverted to a short circuit through the 1/4wave coax,
if he was tuning for 1/2wave he would short circuit the far end so that short will be repeated as a short circuit with no reactance at the analyzer when the coax is an electrical 1/2wave.
He's tuning a 1/4wave coax jumper for no reactance, the near infinite open circuit impedance at the far end is inverted to a short circuit through the 1/4wave coax,
if he was tuning for 1/2wave he would short circuit the far end so that short will be repeated as a short circuit with no reactance at the analyzer when the coax is an electrical 1/2wave.
I was looking for a YouTube demonstration of the idea you mentioned above. I posted the wrong link Bob.
I was hoping to find a video done by MFJ, and I found a boat-load of coax discussions and demonstrations, but nothing from MFJ that really discussed making tuned 1/2 wavelength lines for antenna tuning.
The video below, more or less, shows the technique you suggested, using a dummy load, a T connector, and in this case an MFJ 249 analyzer.
I was hoping to find a video done by MFJ, and I found a boat-load of coax discussions and demonstrations, but nothing from MFJ that really discussed making tuned 1/2 wavelength lines for antenna tuning.
The video below, more or less, shows the technique you suggested, using a dummy load, a T connector, and in this case an MFJ 249 analyzer.
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