Home |
Search |
Today's Posts |
#1
![]() |
|||
|
|||
![]()
Of all the HF antennas one might reasonably make out of wire or
aluminum tubing, which produces the strongest groundwave? Ground-mounted quarter-wave vertical? Horizontal dipole at a height of 1/4 wavelength? Or what? I'm sure someone must have experimented, or done computer modelling, to seek an answer to this question but I am not finding any information. -- -30- |
#2
![]() |
|||
|
|||
![]()
On Sep 29, 8:39 pm, Anonymous wrote:
Of all the HF antennas one might reasonably make out of wire or aluminum tubing, which produces the strongest groundwave? Ground-mounted quarter-wave vertical? Horizontal dipole at a height of 1/4 wavelength? Or what? I'm sure someone must have experimented, or done computer modelling, to seek an answer to this question but I am not finding any information. -- -30- If all things are set up properly, normally the 5/8 will give the strongest ground wave for a single element. You have very little if any true ground wave from a horizontal antenna. You do have a space wave, but it's not quite the same. MK |
#3
![]() |
|||
|
|||
![]()
On 30 sep, 03:39, Anonymous wrote:
Of all the HF antennas one might reasonably make out of wire or aluminum tubing, which produces the strongest groundwave? Ground-mounted quarter-wave vertical? Horizontal dipole at a height of 1/4 wavelength? Or what? I'm sure someone must have experimented, or done computer modelling, to seek an answer to this question but I am not finding any information. -- -30- Hello "Anonymus". I assume that you want to maximize field strength close to ground. At the low HF (and of course medium wave), the surface wave contributes lots to the signal strength. These ones you can only excite with a vertical polarized antenna. Also elevated vertical antennas induce surface waves, so a ground connection is not mandatory (but many times required practically because of antenna size). As mentioned by other poster, 5/8lambda vertical fed over a good ground network gives highest gain in horizontal plane (with a minor side lobe). At high frequency (and low ground conductivity), surface waves do not play an important role. In that case height is the determining factor (with high directivity in the horizontal plane). Polarization is of less importance. Best regards, Wim PA3DJS |
#4
![]() |
|||
|
|||
![]()
Wim mentioned the high angle lobe you get with a 5/8 wave antenna. It
produces a sky wave which can, if skip conditions are present, interfere with the ground wave signal and cause fading. This could be a very common and serious problem at higher frequencies where the ground wave is weaker and the sky wave stronger. You can reduce this by keeping the radiator height to 1/2 wavelength or less. Antennas in that height range don't have a high angle lobe, although they do produce some radiation at high angles. Radiators near a half wavelength in height produce less high angle radiation than shorter antennas, and they have more horizontal gain, so I'd think they would be best. I'm sure the AM broadcasters have other "anti-fading" techniques -- other newsgroup contributors would know much more about them than I do. Roy Lewallen, W7EL |
#5
![]() |
|||
|
|||
![]()
On Sep 30, 5:40 pm, Roy Lewallen wrote:
Wim mentioned the high angle lobe you get with a 5/8 wave antenna. It produces a sky wave which can, if skip conditions are present, interfere with the ground wave signal and cause fading. This could be a very common and serious problem at higher frequencies where the ground wave is weaker and the sky wave stronger. You can reduce this by keeping the radiator height to 1/2 wavelength or less. Antennas in that height range don't have a high angle lobe, although they do produce some radiation at high angles. Radiators near a half wavelength in height produce less high angle radiation than shorter antennas, and they have more horizontal gain, so I'd think they would be best. I'm sure the AM broadcasters have other "anti-fading" techniques -- other newsgroup contributors would know much more about them than I do. Roy Lewallen, W7EL One way to get more ground wave without introducing the high lobes is to use a collinear vertical. Just how practical this is depends on the frequency of operation and what you have to hold the antenna up. For example, on 30MHz, you could pretty reasonably put a half-wave above a half-wave or above a quarter wave. On 3.5MHz, that would be much more difficult for most of us. Even on 20 meters, you might be able to do a collinear vertical. I suppose that MW AM broadcasters are unlikely to use a vertical collinear, but maybe one of the other contributors with lots of broadcast antenna experience could comment on that. Cheers, Tom |
#6
![]() |
|||
|
|||
![]()
"K7ITM" wrote
I suppose that MW AM broadcasters are unlikely to use a vertical collinear, but maybe one of the other contributors with lots of broadcast antenna experience could comment on that. ___________ Almost all US AM broadcast stations use non-sectionalized vertical monopoles, but a few use a form of the Franklin antenna comprised of two collinear, end-fed verticals. The current thought on this in broadcast circles is that the additional h-plane gain that provides over a 195-degree non-sectionalized monopole isn't worth the initial cost and the continuing need to check and adjust the power division and phasing between the sections of a Franklin. RF |
#7
![]() |
|||
|
|||
![]()
Anonymous wrote:
"Of all the HF antennas one might reasonably make out of wire or aluminum tubing, which produces the strongest groundwave?" Vertical antennas make the strongest groundwaves. Assuming you are confined to a single ground-mounted element of reasonable height and complexity, Kraus in the 3rd edition of "Antennas" gives a simple answer in Figure 16-35 on page 569. A 0.64-wavelength tall vertical radiator produces the greatest field strength at one mile in the direction of the horizon. Best regards, Richard Harrison, KB5WZI |
#8
![]() |
|||
|
|||
![]()
"Richard Harrison" wrote
Kraus in the 3rd edition of "Antennas" gives a simple answer in Figure 16-35 on page 569. A 0.64-wavelength tall vertical radiator produces the greatest field strength at one mile in the direction of the horizon. _____________ ....and subparts (c) and (e) of that figure clearly show the high-angle sidelobe centered at about 58 degrees elevation from an 0.64-lambda monopole that may cause self interference -- per Roy Lewallen's post earlier in this thread. RF |
#9
![]() |
|||
|
|||
![]()
Eichard Fry wrote:
"...and subparts (c) and (e) of that figure clearly show the high-angle sidelobe cebtered at about 58 degrees elevation from an 0,64-lambda monopole that may cause self interference..per Roy Lewallen`s post earlier in this thread." All too true. In AM broadcasting, at night the sky wave may interfere with the ground wave beyond the skip zone. In HF transmissions, the ground wave is likely exhausted in short order so that the sky wave is the only signal available at considerable distances. There will be no ground wave to beat with the sky wave to cause fading but there are numerous paths for the sky wave which produce plenty of fading without adding any ground wave to the mix. Best regards, Richard Harrison, KB5WZI |
Reply |
Thread Tools | Search this Thread |
Display Modes | |
|
|
![]() |
||||
Thread | Forum | |||
Looking for groundwave propagation experience on 20m | Antenna | |||
Homebrew-Apt SWL Antenna | Shortwave | |||
Homebrew antenna | Antenna | |||
Homebrew HT antenna | Antenna | |||
homebrew car antenna | Homebrew |