I understand that the radius of the elements have an important part for designing the antenna; my question is:
If increasing the diameter, does it make it more broadbanded? I built a quarter wave a few months back, and experimenting with larger diameter elements, it appeared to make it cover a larger segment of the 2 meter band.
I was going to ask if there is a trade-off using larger diameter and answered my own question; Yes weight!
I'll be back with some more questions soon, (maybe too soon! lol)
edit:
I forgot to ask if you have used the 4NEC2 application for designing any of your antennas. I've only read a very small amount of info concerning that program. Just wondering if it is useful to the small time antenna experimenter. (just checked eHam and they have praise for it):
4NEC2 freeware antenna modeller and optimizer Product Reviews
Thanks for being patient.
Roger,
Sorry for the delay, but I was out tending my horses and didn't see your question right away. Anything interesting happen while I was gone? Yea, I didn't think so. :roll:
Yes, antenna weight can be an issue, if you are using solid elements. At VHF and UHF most of your currents are going to be running on the surface of the elements, so there is really no good reason to use solid elements. If you use hollow tubing, the antenna can be very light weight.
As far as bandwidth, if you ask 10 people what their definition of bandwidth is, you will probably get 10 different answers. So you have to pick a reference point or your reasoning may not make any sense.
I tend to look at bandwidth as the frequency span where the SWR is less than 2:1. This is often near the value where auto tuners stop working and transmitters start backing off the power. Some like to use a lower value. I don't use an autotuner, so I can usually work with a higher SWR. But it shouldn't matter as long as everyone knows your reference point.
So, yes, element diameter does affect bandwidth. The bigger the diameter of the dipole elements, the wider the bandwidth. But don't expect the differences to be huge. Some time ago, I was curious about this myself so I assumed a Air Band Dipole and ran some simulations using various diameters from #14 AWG (1.6277mm) wire to 1.00" (25.4mm). The results were interesting and I documented it with a web page at:
Martin E. Meserve - K7MEM - Air Band Antenna Analysis
It's not a very long page, but at the bottom of the page is a graph and the buttons above the graph allow you to show the SWR and Gain for each of the diameters I tested. The dipole lengths were adjusted slightely for each diameter, as the diameter also affects the resonance. But you can clearly see that a
#14 AWG wire might give you a 2:1 SWR bandwidth of 11 MHz. But if you increase that to 1.00" diameter, the 2:1 SWR bandwidth increases to 22 MHz. That's a substantial increase in bandwidth, but it's also a substantial increase in element diameter. If you looked at it over a smaller increment, say 1/4", the change would not be as large.
Now that was just a simple dipole. A yagi might be a little bit different because there are other things to consider. With a yagi you might have a metallic boom and you have parasitic elements very close to the driven element. All of these affect the parameters of the antenna.
A general rule of thumb for yagis is, the bandwidth is about 7 pecent of the operating frequency. So a yagi for 2 meters should provide a 2:1 SWR bandwidth of 10 MHz. That should be more than enough to cover the 2 Meter band. But if your transmitter backs off the output at a lower SWR, you may not get as much coverage. But you also have to think about how much of the band you use. If your only working FM (Simpled and repeaters) you don't really need coverage below 146 MHz. However, if your doing SSB, CW, or Satalite work, it may be handy.
Also, yes, 4NEC2 is a good application. But it is more intended for analysis then design. I use a antenna simulator that I put together years ago. It's written in C and I run it under Cygwin. Under Cygwin I can build shell scripts that will do all the work for me. I use my simulator to collect the data for the previously mentioned web page.
I used the simulator to simulate multiple 2-element 10 Meter yagis. Half of the antennas had a single driven element and one reflector element. The other half had a single driven element and one director element. The results were very interesting
Overall, antennas are a very big subject. That is why so many hams spend huge amounts of time trying to understand them. It try to understand them to some extent, but I'm really just interested in making them work to my satisfaction. Then I can work on other aspects of radio.
Martin - K7MEM
