Dipole Construction

[invergordon]

I'm designing and building a horizontal wire receiving dipole and would like some advice on a couple of things that are not clear in my mind.

When constructing the wire elements and using the standard dipole calculations that specify say, 8.03 metres for each leg, is that the length from one end of an insulator to:

a. The centre point of the T centre insulator,or
b. The end point of the T centre insulator, or
c. The actual length of the wire including the turns at each insulator

Currently I have it temporarily set at point B so in effect I have an additional 70-80mm through the insulator and crimped and secured to the main length.

If the consensus is that point B is correct, how much additional wire can be used before the calculated length is affected and resonance lost?

Thanks for any comments in advance.

Michael
New Zealand
hfaero.com

 

[chrissim]

If I remember correclty (I've been spoiled by online calculators), it's 468/frequency, which gives you the total length of the element. However, I have used the following:

Amateur Dipole Antenna Calculator

You can pick total length or length per side.

For a dipole, each side should be approximately equal in length as it transitions from the feedpoint, although I'm sure I have snipped off a fraction less on one end than the other when tuning for lowest SWR. I try to keep the absolute ends secured to the insulators the same length. I always cut a bit longer on each side to account for wrapping the element(s) around the insulator at the ends.

So then, if I am using the link I provided, I pick calculate one side for 7.200. It returns a value of roughly 32 ft 6 inches (9.906m). I cut it for 33 feet both ends, connect the elements to a 1:1 choke (debatable), and then connect the ends to the insulators. I then raise it to operating hight and note the SWR. I bring the antenna down and cut a small portion from the insulator side on each end until the values are where I want them. Takes me about 10 minutes per antenna.

I apologize for including information you already know, but included it anyhow for completeness. Hope I helped.

 

[invergordon]

Thanks chrissim, so from this response and one other off forum I'm hearing that the additional length to secure the end insulators and centre T are not a factor in the leg calculation.

Not having a transmitter or antenna analyser is there any other means to test resonance at the required frequency and harmonics other than by just listening and assuming?

 

[Blackswan73]

For a receive only antenna, the slight difference is not going to matter. The slight variation in length is only going to move the resonant frequency a megahertz or two. Not enough to really notice a difference. If you want even more bandwidth, look at a terminated tilted folded dipole. Same overall length, but much broader reception.

 

[invergordon]

Thanks muskrat39, what do you reckon a "slight variation" is? In my world on HF a shift of a megahertz or two would be disastrous! I'm looking at no more than a +/- 80kHz either side of a nominal 8.890 MHz.

 

[majoco]

8.890MHz, ¼wave less 5% = 8.014metres
8.900MHz, ¼wave less 5% = 8.005metres.

Therefore 10kHz change in frequency corresponds to 9mm length change - theoretically.

BUT what is the "Q" or bandwidth of the antenna? Does the signal strength fall off rapidly as you get away from it's so-called 'resonant' frequency? I don't think you would notice 100kHz either side for reception, transmission maybe.

IMHO the 'resonant' frequency will be more affected by the local environment, height, ground conductivity, metal roof etc. far more than any small change in the antenna length.

The answer is to 'suck it and see' - one test is worth a thousand opinions!

 

[invergordon]

I can't verify right now but I saw the noise floor rise and peak at about 8.5MHz when viewed with 6MHz of available bandwidth. I'll make a significant change when I have sufficient daylight and see where that noise floor peaks which should tell me if it is location/environment induced or actual antenna changes.

One thing I have noticed from last week is a drop off signals from Europe, Gander was not as strong Sunday/Monday night as last week so the propagation changes just compound the testing....darn!

 

[WA0CBW]

Resonance is more important for transmitting than receiving. Resonance is the point at which the inductive and capacitive reactances are equal. Power cannot be dissipated in reactances so cancelling them out of the antenna leaves only the resistive portion of the antenna to radiate RF. Also resonance does not mean a 1 to 1 SWR. The resistive portion of the antenna could be 25 ohms or 100 ohms and still be resonant with an SWR greater than 1. And as mentioned above the environment plays a large roll in the actual impedance of the antenna.

Technically the antenna length for a particular frequency is the distance from the end of the wire at one leg of a dipole to the other end of the leg where it connects to one side of the feedline. The distance between the legs at the center of the dipole (usually less than 3 inches) is negligible at frequencies below 30 MHz. But as we said many other factors contribute to the actual length (size of wire, height above ground, proximity of other objects, etc.)
BB

 

[SCPD]

I'll make a significant change when I have sufficient daylight and see where that noise floor peaks which should tell me if it is location/environment induced or actual antenna changes.

The most important thing you can do with a receiving dipole is get it up high. If you really want to hear signal it needs to be 60ft or higher. The higher the better.

The only downside is that a dipole is still somewhat directional. Ironically, the less height, the less directional -- but less signal. So ideally, you'd want one running N-S and E-W at maximum height.

Here's a nifty chart of dipole patterns (viewing from the side) when you take height into consideration. As you can see, the minimum height needed is half-wavelength (of your target frequency.)

Example: @ 11 megs, it's 13m.

 

[invergordon]

Thanks Nick, I will erect the dipole to a max height of 7 maybe 8m but even half the wavelength of 8.855MHz is a lofty 16.5m, just not doable for my neighbourhood. So the best I'm going to get will be Height = 0.21 x wavelength.

Thanks for the info and chart, good food for thought!

 

[SCPD]

Thanks Nick, I will erect the dipole to a max height of 7 maybe 8m but even half the wavelength of 8.855MHz is a lofty 16.5m, just not doable for my neighbourhood. So the best I'm going to get will be Height = 0.21 x wavelength. Thanks for the info and chart, good food for thought!

Sure no problem. Yes this is the main reason why I can't use a dipole either.

 

[majoco]

I don't know if if the display of noise across the spectrum is a good test for the bandwidth of an antenna - you would have to be very careful that the receiver has a very flat frequency response from say 100kHz to 30MHz - I suppose you could inject a signal from a good signal generator with known flatness at every 500kHz!

I took a pic of my G33DDC listening to Radio Helleniki connected directly to my OCFD at 0415z. You can see on the main wideband display that the background noise level is fairly constant at around -105dBm from say 6 to 20Mhz. Tha narrow band signal however while clearly showing the carrier and sidebands also shows that the background noise is around -125dBm! The narrow band receiver certainly is not letting so much noise in. It' not the AGC turning the gain down using the strong carrier from R.Helleniki as tuning off the station still leaves the noise at -125dBm.

The neighbours haven't turned on their plasma TV yet!