Dipole length

Uksoundz · Post by **Uksoundz** » Thu Aug 06, 2015 5:36 pm

Hi,

I've just ordered a dipole from eBay to be cut to 91.8, anyway the total length when out together it exactly 160cm which I assume means it hasn't been cut cause I believe it should be shorter. Can anyone let me know if this is the case please?

Also I've bought the below:
http://pages.ebay.com/link/?nav=item.vi ... ID=EBAY-GB

It doesn't mention anything about a balun, do I need to create one?

Thanks guys

pjeva · Post by **pjeva** » Thu Aug 06, 2015 6:24 pm

Without balun, you will have about 1: 1,5 swr at best. But this is acceptable. You will need ferrite beads at cable close to connector to prevent coax radiation and field pattern distortion. Also, i would not recommend 500w into this connector

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Uksoundz · Post by **Uksoundz** » Thu Aug 06, 2015 7:00 pm

Ok, could I roll up the coax 5-7 times (round like a coke can or something) I'm sure I've heard that somewhere but most probably have made that up!!

Also am I right in thinking the total length for 91.8 would be 152cm, this is from end to end including the middle bit?

Analyser · Post by **Analyser** » Thu Aug 06, 2015 8:33 pm

Uksoundz wrote: Also am I right in thinking the total length for 91.8 would be 152cm, this is from end to end including the middle bit?

Yes.

pjeva · Post by **pjeva** » Fri Aug 07, 2015 3:40 pm

It needs to be a bit shorter. Calculate lenght from tip to tip using formula:
286 / f * 0.95 / 2 -> dipole lenght in meters

Where f is your frequency in MHz
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muttley · Post by **muttley** » Sat Aug 08, 2015 12:48 am

[quote="Uksoundz"]Hi,

Also I've bought the below:
http://pages.ebay.com/link/?nav=item.vi ... ID=EBAY-GB
Try
http://www.ebay.co.uk/itm/Dipole-Aerial ... 19f01a3ca5, It should work abit better for you then the other link,

Analyser · Post by **Analyser** » Sat Aug 08, 2015 9:40 am

pjeva wrote:It needs to be a bit shorter. Calculate lenght from tip to tip using formula:
286 / f * 0.95 / 2 -> dipole lenght in meters

Where f is your frequency in MHz
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The formula used by the OP is a very common one. Can you explain where you get the 286 from?

teckniqs · Post by **teckniqs** » Sat Aug 08, 2015 10:38 am

Just use this website and type your frequency in, great for building directionals too!

http://www.csgnetwork.com/antennae3ycalc.html

pjeva · Post by **pjeva** » Sat Aug 08, 2015 11:01 am

Analyser wrote:
The formula used by the OP is a very common one. Can you explain where you get the 286 from?

Sure. Actual speed of light is 286000 m/s not full 300000. It is not much of a difference and it is quite common to use 300 and then cut elements to the right size... In practice with about 15- 20mm diameter pipes, you get almost perfect resonance with my formula above.

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Analyser · Post by **Analyser** » Sat Aug 08, 2015 12:24 pm

I thought the speed of light was more than that! Just googled it and it says 299 792 458 m/s.
Where did you get the 286 from, that's a lot different.

pjeva · Post by **pjeva** » Sat Aug 08, 2015 2:36 pm

Analyser wrote:I thought the speed of light was more than that! Just googled it and it says 299 792 458 m/s.
Where did you get the 286 from, that's a lot different.

Sorry, i made mistake but only in units. It is 286000 km/s
299... is speed of light in vacuum. Since we are broadcasting in atmosphere, it is a bit slower. Just like we take velocity factor of coax into account. Velocity factor IS actually speed of light through some material (like PVC or teflon). Light would not actually travel through pvc, but it is approximate calculated value.

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Analyser · Post by **Analyser** » Sat Aug 08, 2015 3:27 pm

pjeva wrote:
Analyser wrote:I thought the speed of light was more than that! Just googled it and it says 299 792 458 m/s.
Where did you get the 286 from, that's a lot different.
Sorry, i made mistake but only in units. It is 286000 km/s
299... is speed of light in vacuum. Since we are broadcasting in atmosphere, it is a bit slower. Just like we take velocity factor of coax into account. Velocity factor IS actually speed of light through some material (like PVC or teflon). Light would not actually travel through pvc, but it is approximate calculated value.

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I understand totally what you mean however I've never seen the speed of light adjusted like that before. Most people take the normal speed of light and make one adjustment for VF of the antenna.
A lot of web calculators assume the VF is a standard 0.95 however Steve Moss used 0.935 and in my own experiments over the years I've found it to be a little lower still (but not a lot).

Anyone else have any input to this? I'd be interested to see what other people think. I'll have a read later on tonight to see if I can find a definite answer.

yellowbeard · Post by **yellowbeard** » Sat Aug 08, 2015 4:42 pm

I read somewhere the reduction in size was to account for the velocity factor in the dipole element, not the feeder. This is different depending on the material and alloy used, the 95% is a rule of thumb from ham radio (copper) long wires.

pjeva · Post by **pjeva** » Sat Aug 08, 2015 6:09 pm

0.95 in my formula is shorting due to "end effect" and not VF of material used. I'm on the road at the moment and i will try to explain this later... Or you can try to google it...

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sharky · Post by **sharky** » Mon Aug 17, 2015 8:09 pm

Uksoundz wrote:Ok, could I roll up the coax 5-7 times (round like a coke can or something) I'm sure I've heard that somewhere but most probably have made that up!!

A coke will radiate but a 40mm section of PVC would be just fine.. It helps to decouple the feedline from the antenna.

shuffy · Post by **shuffy** » Mon Aug 17, 2015 10:42 pm

sharky wrote:A coke will radiate but a 40mm section of PVC would be just fine.. It helps to decouple the feedline from the antenna.

I assumed he meant take the coke can out after using it to make the coil!! Then just use a couple of small cable ties to hold the loops together.

pjeva · Post by **pjeva** » Mon Aug 17, 2015 11:09 pm

shuffy wrote:
sharky wrote:A coke will radiate but a 40mm section of PVC would be just fine.. It helps to decouple the feedline from the antenna.
I assumed he meant take the coke can out after using it to make the coil!! Then just use a couple of small cable ties to hold the loops together.

Exactly. 4-6 cm plastic tube would be ok.

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