Need help with reading a tape mesure

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nay27uk
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Need help with reading a tape mesure

Post by nay27uk » 29 Jan 2018, 11:56

Right trying to make a fan dipole and measurements are critical.

I have never been good at reading a tape measure so I want to now exactly "how many of the little lines between 1 foot" are the following measurements.

.545 foot
.64 foot
.85 foot
.585 Foot
.785 Foot
.405 Foot

Would realy like to know in how many of the little lines between 1 foot

Sorry but maths and measurements is not my strong point.

Measurements for the Inverted V Fan Dipole are as below I am just struggling with the ".sometings" of feet so need to know the above going by the little lines between a foot on a tape mesure.

80M
Each leg of the Inverted Vee is 61.545 Feet

40M
Each leg of the Inverted Vee is 31.64 Feet

20M
Each leg of the Inverted Vee is 15.85 Feet

15M
Each leg of the Inverted Vee is 10.585 Feet

10M
Each leg of the Inverted Vee is 7.785 Feet

6M
Each leg of the Inverted Vee is 4.405 Feet
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Re: Need help with reading a tape mesure

Post by Tigersaw » 29 Jan 2018, 12:32

There is no point in being so critical - the exact exact length depends on the wire material, diameter, tension, velocity factor, temperature, height above ground, end effects from trees or buildings, interaction, things, things,
Best just cut it a bit too long then fold back/trim off when you put it up

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Re: Need help with reading a tape mesure

Post by nay27uk » 29 Jan 2018, 13:10

Yeah 3 flores up.
3 story block of flats.
Approximately 55 feet from ground level to roof peak.
will be in a communal atic inverted V configuration.
Coax is RG8 Mini approx 5-7 meter run to my Firsat floor flat.
wire is the SotaBeams Stealth antenna wire 100M (brown) https://www.sotabeams.co.uk/antenna-wir ... ight-100m/ so 1.2mm.
Tension will be an inverted V right around one of the attic roofing strut things, the 80m ones being cable clipped to the roof strut thingy and bent back on itself where necessary to fit the space.
Velocity factor DON'T KNOW.
Temperature in winter no more than 35c and summer no more than 45c estimated.
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Re: Need help with reading a tape mesure

Post by paulears » 29 Jan 2018, 15:51

There's also a great rule for this - never specify decimal points greater than you can measure - but if you ever do it again, do it in metric.

Old fashioned rulers have loads of scales. That's why metric is simpler. If the ruler has the little lines in 10ths of an inch, then it's easier, .5 of a foot is 6", and your .545 is 6 and a half inches!

Inches, feet and yards are just daft things to even try to work with.

For receive use, I'd take your calculated measurements and do this.
80M
Each leg of the Inverted Vee is 61.545 Feet 61' 6"
40M
Each leg of the Inverted Vee is 31.64 Feet. 31' 7"
20M
Each leg of the Inverted Vee is 15.85 Feet. 15' 10"
15M
Each leg of the Inverted Vee is 10.585 Feet. 10' 7"
10M
Each leg of the Inverted Vee is 7.785 Feet. 7" 9"
6M
Each leg of the Inverted Vee is 4.405 Feet. 4' 5"

Probably close enough to make little difference.

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Re: Need help with reading a tape mesure

Post by Kaos » 29 Jan 2018, 16:41

Whats wrong with Metric?

I would have thought a quick conversion into metres and centimetres on google would have made the job easier?

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Re: Need help with reading a tape mesure

Post by nay27uk » 29 Jan 2018, 21:03

Thanks paulers I will use that what you put, it is for TX also, so do I just add a few more inches to what you put above as you said your modified measurements are for Receive only.

It was an inverted V calculator online that gave me those measurements, with the Center Frequency of each band imputed for the calculations, so nothing to do with myself lol.

Thanks 2E0LIG
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Re: Need help with reading a tape mesure

Post by paulears » 30 Jan 2018, 21:39

Damn - I replied but lost it, so I'll try again.

Cut the lengths at least an inch or too long. The maths to calculate them exactly need the velocity factor of the cable taking into account, plus the physics of these antennas also messes it up a bit. Each dipole has a low impedance on it's cut frequency - to let it match the radio output, but those other wires will be high impedance, which loads the antenna as a whole slightly. Exactly how much depends on their closeness and the maths is too damn hard for me.

In practice, you just slap a VSWR meter on it and trim. I'd tie a knot say two inches before the far end of each and support them from this, leaving the end floating. Trim the ends a tiny amount at a time. ¼" would be a good start. If your meter is accurate you will see the VSWR dropping with each chop at both ends. At some point it will not change. Stop there. I'd bet you won't get much better than 2:1 - and if you get this you're doing well. It will be a pain to do tuning like this because ideally you need a constant tone so you drive the TX the same each time. I hope it works for you, but my experience of these was that tuning is like balancing one football on top of another. You tune one, and it's good, but then adjustments to another bugger up the first, because ham bands are all mathematically linked. If one of the other dipoles accidentally starts to get close to a multiple of one of the others, or other maths related lengths, it's impedance drops, and impacts on the VSWR. For receive, it doesn't matter, but for TX you might never get a decent match.

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Re: Need help with reading a tape mesure

Post by nay27uk » 30 Jan 2018, 22:55

Cool thanks for the info pal
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