Don’s Antenna talks by N4DJ

  The radiation angle of a horizontal antenna is determined by the height above ground. Below is a table giving height in feet corresponding to the height in wavelengths at 1/4, 1/2, 1, 1 1/2 and 2 wavelengths including the radiation angle for each height. I have also included the formula used to calculate the angle.  The wavelength in feet for any frequency is found by dividing 984 by the frequency in MHz. The radiation angle is arcsin (0.25/ height in wavelengths )

  Antennas systems have gain and loss. In general 3 dB gain is equivalent to doubling your power and - 3 dB gain or 3 dB loss is reducing your power in half. Usually the loss is in the feed-line and any other component connected to the feed-line. .   Let’s assume your transmitter is capable of delivering 100 watts of output power and we will go through what can happen to that power. First of all we need to get the 100 watts into the feed-line or coax heading toward the antenna. Most radios need to have a load connected that is a match for the fixed output circuit of the transmitter. That will usually happen if the system SWR is 2:1 or less. If not some sort of tuner can be used to perform the required matching to whatever the impedance is at the coax. There could be a loss of power at this point if not done properly. After that there will be a loss of power in the coax. This loss is determined by the characteristics of the coax and the length of the coax. The loss will b...

 I will describe a very good antenna for 160 meters, the Inverted L. The inverted L comes in many configurations, some better than others. The Inverted L that I use is not a new antenna, on the contrary, it was described to me many years ago by a man called Mr. 160 meters, W1BB.  An inverted L is simply a top loaded vertical. Since it’s is nearly impossible to get a horizontal antenna high (in terms of wavelength ) on 160 meters it is necessary to use a vertical radiator to get low angle radiation. A quarter wave vertical with a extensive radial system has been a good 160 meter antenna. However it has some limitations. First of all a 130 foot vertical is not easy to install and it has the maximum current point right at the base or ground. It is always best to have the high current point higher above the ground.  The inverted L has a vertical portion somewhat less than a quarter wavelength, maybe 40 to 65 feet. It could be somewhat more or even less if necessary. The rest ...

 This is a real time MUF chart. The numbers in the circles indicate the highest frequency that will refract a radio wave for a 3000 KM or 1900 mile path.  If we want to work 3000KM then we should use a frequency at or below the MUF at the midpoint. On the map below we see that the MUF at the midpoint between Virginia and California is above 21 MHz. The mid point is actually around 30 MHz. This means we can be pretty sure to be able to make a contact out to 3000 KM on probably both 21 and 28 MHz.  Since California is a bit over 3000 KM the single hop angle will be lower than the angle needed for 3000 KM.  This is the reason that you can almost always work longer than 3000 KM at the MUF for 3000 KM and usually not closer than 3000 KM.  I have drawn two lines drawn on the indicating where I was heard on 21 MHz at the same time I copied the MAP. I was heard in both California and Europe on 21 MHz, but I was only heard in Europe on 28 MHz.   Here is a link for t...

  SWR bandwidth (High Frequency antennas)   In general most radios work perfectly well if the SWR they see is 2:1 or less. In addition to satisfying the transmitter the losses in coax are not going to be excessive with a 2:1 or better SWR. Most of the time I will be talking about dipole antennas to keep it simple. Other antennas are very much the same but there are exceptions.  The 2:1 SWR bandwidth is simply how many kilohertz are between the low 2:1 SWR point on an antenna to the next 2:1 point. Starting with my 80 meter dipole, when set for CW it has a SWR of 2:1 below the bottom of the band at 3,435 kHz. The SWR then goes down to a minimum at 3,535 and back up to 2:1 at 3,635.  The bandwidth between 3,435 and 3,635 is 200 kHz. So my dipole has a 2:1 SWR bandwidth of about 200 kHz.   When tunes for the phone band it has a 2:1 at about 3,608, minimum SWR at 3,708 and again a 2:1 SWR at about 3,825. This bandwidth being just over 200 kHz using these number...