Hi,
I have built a Dipole for 17 Meter Band 18.103 Megs. I calculated the Driven element to be 4.05 Meters (1side of Dipole). But when testing it I have found that to get it to resonate I had to shorten it to 3.69 Meters(1 side of dipole) and it resonates with 1.2 SWR. My question is that if I want to make a 3 element Yagi +5% and -5% for the other 2 elements which value do I use to calculater the Dir and Refl Elements? The calculated one or the measured one. The element is Direct feed with a 1 to1 Balun and the elements are about 15cm in Diameter and the boom will be 3.65 Meters Long. The calculated is much longer than the measured! Why is this happening? Thr Gap between the 2 sides of the Dipole is 127mm(5 inches) . ( I am using parts from a discarded TA33Jr Beam why the spacig is like this) I would like to build the rest of the antenna but want to be sure that it works properly like Gain and F/B Ratio. In Modeling I get no results because it says the element should be 4.05 Meters(not 3.69 Meters more than 1 foot on each side shorter) so modeling does not work its reports it is not resonant but it definely is 1.2 swt at 18.100 Mhz. Can someone please explain to me whats going on? Is the large Gap between the 2 parts of the Dipole causing this shortening issue? is there a way to calculate the effect of this Gap? I t does not make sense to me why it is so far from calculated value. All elements are 15mm in Diameter, the Boom is 50mm and the boom is 3657 mm Long. Any advice or explanation is welcome. My email is vk3xz@tpg.com.au 73 Mike Vk3xz
17 Meter Dipole Help Needed Please
Re: 17 Meter Dipole Help Needed Please
Mike,
A few things.
(1) The spacing will add a small extra distance
(2) You don't say at what height or in which manner you are measuring the dipole - ground /proximity effects may be evident
(3) You don't say what type of 1: 1 balun you are using - some baluns you need to add the length of the balun winding to your calculations.
Here is a quote from the Radio Works 1: 1 Y1-5K balun: "Note: In many installations, the leads exiting the balun's case are part of the antenna and the length must be taken into account. This is especially important on 15, 10 and 6 meters where the antenna length is short and the baluns output leads make the element longer, lowering the resonant frequency. Of course, this is true of all baluns and Line Isolators."
Hope this assists.
Cheers
Peter VK3QI
A few things.
(1) The spacing will add a small extra distance
(2) You don't say at what height or in which manner you are measuring the dipole - ground /proximity effects may be evident
(3) You don't say what type of 1: 1 balun you are using - some baluns you need to add the length of the balun winding to your calculations.
Here is a quote from the Radio Works 1: 1 Y1-5K balun: "Note: In many installations, the leads exiting the balun's case are part of the antenna and the length must be taken into account. This is especially important on 15, 10 and 6 meters where the antenna length is short and the baluns output leads make the element longer, lowering the resonant frequency. Of course, this is true of all baluns and Line Isolators."
Hope this assists.
Cheers
Peter VK3QI
Re: 17 Meter Dipole Help Needed Please
Inverted V calc here http://www.csgnetwork.com/antennaeivcalc.html
now the v makes the dipole closer to 50Ohm , a flat element is less , possibly 25>30 ohms so you need a matching device like a beta , hairpin or gamma match or bend the elements into a v for match , a match does not prove resonance , hope this helps
now the v makes the dipole closer to 50Ohm , a flat element is less , possibly 25>30 ohms so you need a matching device like a beta , hairpin or gamma match or bend the elements into a v for match , a match does not prove resonance , hope this helps
Re: 17 Meter Dipole Help Needed Please
hi Mike, there are lots of variables for calculating dipole length. 18.1Mhz by the formulae should be about 3.937M per side.But the formulas are normally for bare copper wire.Insulated wire and aluminium can be slightly different lengths due to velocity factor.Also if I remember correctly the feed impedence of a dipole is just over 70 ohms so to get a 1:1 swr at 50 ohms you are actually changing the length of the dipole to achieve this.Your 5 inch gap will also cause a slight variation to the lengths of your wires.Also the height above the ground has an effect on the antenna swr as well.
I just made a fan dipole for 40M,30M and 20M and had to do some trimming to get it right.
regards Wayne VK4WTN
I just made a fan dipole for 40M,30M and 20M and had to do some trimming to get it right.
regards Wayne VK4WTN
Re: 17 Meter Dipole Help Needed Please
My experience while building a lot of yagis over a lot of years is that the construction determines the "End Effect" (Ee) multiplier value.
The commonly used value for wire dipoles varies between about 0.92 and 0.95 for insulated and non-insulated wire types but it is often lower than that when larger diameter materials are used for the construction (your "All elements are 15mm in Diameter" is telling). Yours currently calculates out at around 0.89 (your 3.69 metres actual versus 4.14 metres free space) but part of that may be the balun wire connection lengths (re-read Peter 3QIs comment). I have taken to using 10mm wide aluminium strip to connect the balun to my dipoles in as short a length as is practicable, even at HF.
The following are valid for a frequency of 18.1 MHz:
4.1436m for a quarter wave in free space. (note: not your 4.05m value, wherever that came from)
3.9364m for a quarter wave with Ee of 0.95
3.812m for a quarter wave with Ee of 0.92
The reflector and director will have the same basic end effect value due to the manner of construction so therefore the +5% and -5% respectively will apply to the shortened length of 3.69m.
The other factor that you really have to take into account is the current height above ground. The closer it is then the lower the frequency (consider it as the capacitance of the element material to the ground plane). Raising it decreases the capacitance and moves the frequency upward. You need to realise also the effect of the reflector and director such that either or both will shift the dipole's frequency but more-so the final feed impedance.
My normal approach with tuning HF yagis is to mount them with the boom vertical with the reflector at least one metre above ground. Hold the boom in place with nylon ropes from the upper end so that it stays close to vertical. Make sure there is nothing overhead - including tree branches. Do your SWR run while mounted that way and plot the overall response. Adjust the driven element so that it centres on the desired frequency. If the basic SWR is still high (>1.5:1) at the centre of the dip then you may need to take the other suggestions on board, a hairpin being the most likely one.
The presence of the reflector and director will cause a change in the feed impedance, not so much the DE frequency, but they have a far greater effect on the gain of the array. You might like to comment on the proposed spacing of these from the driven element (eg. about 0.15 wavelength for Ref-DE and 0.1 wavelength DE-Dir) with your available boom length.
I started playing with a 12m/17m trapped dipole (using 16mm and 10mm tubing telescoping pieces) early last week with the view of adding a reflector and eventually mounting these two new elements on the Cushcraft A3S tribander boom. Adding the trap each side made for interesting observations on DE frequency shifts. Unfortunately the rain over the last weekend curtailed my experiments but I hope to get back to it within a few days.
73 Doug VK4ADC
The commonly used value for wire dipoles varies between about 0.92 and 0.95 for insulated and non-insulated wire types but it is often lower than that when larger diameter materials are used for the construction (your "All elements are 15mm in Diameter" is telling). Yours currently calculates out at around 0.89 (your 3.69 metres actual versus 4.14 metres free space) but part of that may be the balun wire connection lengths (re-read Peter 3QIs comment). I have taken to using 10mm wide aluminium strip to connect the balun to my dipoles in as short a length as is practicable, even at HF.
The following are valid for a frequency of 18.1 MHz:
4.1436m for a quarter wave in free space. (note: not your 4.05m value, wherever that came from)
3.9364m for a quarter wave with Ee of 0.95
3.812m for a quarter wave with Ee of 0.92
The reflector and director will have the same basic end effect value due to the manner of construction so therefore the +5% and -5% respectively will apply to the shortened length of 3.69m.
The other factor that you really have to take into account is the current height above ground. The closer it is then the lower the frequency (consider it as the capacitance of the element material to the ground plane). Raising it decreases the capacitance and moves the frequency upward. You need to realise also the effect of the reflector and director such that either or both will shift the dipole's frequency but more-so the final feed impedance.
My normal approach with tuning HF yagis is to mount them with the boom vertical with the reflector at least one metre above ground. Hold the boom in place with nylon ropes from the upper end so that it stays close to vertical. Make sure there is nothing overhead - including tree branches. Do your SWR run while mounted that way and plot the overall response. Adjust the driven element so that it centres on the desired frequency. If the basic SWR is still high (>1.5:1) at the centre of the dip then you may need to take the other suggestions on board, a hairpin being the most likely one.
The presence of the reflector and director will cause a change in the feed impedance, not so much the DE frequency, but they have a far greater effect on the gain of the array. You might like to comment on the proposed spacing of these from the driven element (eg. about 0.15 wavelength for Ref-DE and 0.1 wavelength DE-Dir) with your available boom length.
I started playing with a 12m/17m trapped dipole (using 16mm and 10mm tubing telescoping pieces) early last week with the view of adding a reflector and eventually mounting these two new elements on the Cushcraft A3S tribander boom. Adding the trap each side made for interesting observations on DE frequency shifts. Unfortunately the rain over the last weekend curtailed my experiments but I hope to get back to it within a few days.
73 Doug VK4ADC
Doug VK4ADC, QG62lg51
http://www.vk4adc.com
http://www.vk4adc.com
Re: 17 Meter Dipole Help Needed Please
Mike,
A 3.65m long boom is too short to make a 3 element Yagi on 18.1 MHz that will match to 50 ohms with a split dipole and 1:1 balun. For that feed arrangement to 'work' and produce a low SWR, the boom length realistically needs to be around 5.5m long. However, a 3 element Yagi on a 3.65m long boom with a split dipole feed would show a low impedance which could be matched to 50 ohms by using a Beta match (same as used on the successful HyGain Yagis of the 1970's and 80's).
With those materials you describe and a split dipole feed with 1:1 balun, one could make a 2 element Yagi on a 2.8m long boom that would have a good match to 50 ohms, show about 10dB F/B ratio, and only be about 1dB less gain than a short 3 element Yagi. Make the Reflector 4.16m each side and adjust the length of the DE to get low SWR. If the SWR does not become 1:1 at 18.1MHz, then there might be "something going on" in the balun ...
73, Bob ZL1RS
A 3.65m long boom is too short to make a 3 element Yagi on 18.1 MHz that will match to 50 ohms with a split dipole and 1:1 balun. For that feed arrangement to 'work' and produce a low SWR, the boom length realistically needs to be around 5.5m long. However, a 3 element Yagi on a 3.65m long boom with a split dipole feed would show a low impedance which could be matched to 50 ohms by using a Beta match (same as used on the successful HyGain Yagis of the 1970's and 80's).
With those materials you describe and a split dipole feed with 1:1 balun, one could make a 2 element Yagi on a 2.8m long boom that would have a good match to 50 ohms, show about 10dB F/B ratio, and only be about 1dB less gain than a short 3 element Yagi. Make the Reflector 4.16m each side and adjust the length of the DE to get low SWR. If the SWR does not become 1:1 at 18.1MHz, then there might be "something going on" in the balun ...
73, Bob ZL1RS
Bob, ZL1RS in the Bay of Islands at RF64vs