3
$\begingroup$

I am trying to engineer my first antenna to get on the air. I need to hit a repeater 50 miles away at a lower elevation than me so that I can use its IRLP nodes. Would this mean I am looking for low angle radiation?

25 feet up with an HT in the clear and a rubber duck antenna and I can hear just the faintest crackle. I believe that a rhombic would be a cheap, interesting, and clear way of getting my signal to the repeater.

Will it need to be vertically polarized, considering everyone else will be? I started working on the design, here are some preliminary radiation maps before optimization. I just wanted to confirm that I had the correct polarization for the job. ANY tips regarding rhombics would be appreciated as I am new to the hobby and will probably learn a few things the hard way on this project :D

* Note that the scale on the perimeter of the chart is shifted 90 degrees for some reason, I didn't see this at first. *

Horizontal Plane General Orientation

$\endgroup$
  • $\begingroup$ If you can hear a crackle with a rubber duck, you might get by with an ordinary 1/4 wave vertical or a 1/2 wave dipole. It should be easy enough to try, before trying some more complicated design. $\endgroup$ – Phil Frost - W8II Jul 1 '15 at 23:44
1
$\begingroup$

Some information, since you asked for comments:

1) your rhombic isn't working properly. It shouldn't have a double beam like that, it should have a single forward beam. Something about the proportions is wrong.

You can and should design a rhombic from formulas, and only simulate at the end. It's a well understood antenna, there is good design data for it.

It needs a termination resistor at the end away from your feed. It's not showing in your model. This needs to be non-inductive, which is tricky for 2m, possibly made of a hundred quarter-watt resistors in series and parallel.

2) as they are usually built, rhombics are horizontally polarised. The repeater is almost certainly vertical. You need to tilt it up on edge to make it vertical. The masts that support it, both the the middle and at the front, must be non-conductive.

3) You're simulating over a ground plane, which is appropriate for HF because it's so intimately involved in the antenna fields. For a VHF antenna 25' up, the ground doesn't matter. You should remove the ground and simulate the antenna in free space. The ground will get involved later, a hundred metres away, but it affects all antennas equally.

4) consider a yagi instead. The main features of rhombics are their simple construction, limited number of supports, and wide bandwidth to cover many bands. This makes them attractive for HF when you have a lot of space. They have reasonably good gain, but nothing like a yagi. Do some digging for information about the gain of a yagi and compare. For example, you can easily make a yagi for 2m from an old broomstick and some fencing wire / welding rods, that will give you about 10 dBi, which is at least 13 to 16 dB more than your handheld antenna yields. If you want a lot of gain, look up the Rope Yagi, which can be made very long.

$\endgroup$
  • $\begingroup$ I am grateful for your pointers! I am having a very hard time finding forumlas for the rhombic. I do know how to calculate how long a wavelength is at a given frequency, which I used to make this rhombic 5.5 wavelengths long. I have posted updated radiation charts that portray the same antenna without the ground plane. That seems to fix the double lobe problem.The reason I am trying a rhombic is because it is bidirectional, if at a resonant frequency, and thus does not need the terminating resistor. I was hoping to use it for the 6m, 2m, 70cm bands because of harmonics. $\endgroup$ – Zeke Legge Jul 1 '15 at 19:15
  • $\begingroup$ What do you think of pages 239-240 in the 1976 ARRL Antenna Book? The lengths that are listed in the table 11-1 seem to be much longer than the 5.5 wavelengths that I chose, which is only 17 meters from point to point. [ARRL Antenna Book](smcelectronics.com/DOWNLOADS/1974-ARRLANT.PDF} $\endgroup$ – Zeke Legge Jul 1 '15 at 19:22
  • $\begingroup$ What do you think would be the optimum dementions for a tri band rhombic as described? I could try it in the antenna program and see how it performed. Other than choosing how many wavelengths long you want each segment to be, the only other thing that is obviously manipulable seems to be the angles. Is there a formula that defines what angle measure that you should use? Sorry, and no pressure if you don't know all of the answers. I am just looking for a bit of help, this is still a bit out of my league!! $\endgroup$ – Zeke Legge Jul 1 '15 at 19:33
  • $\begingroup$ The ARRL antenna book gives a rhombic for 14 to 28 MHz, with a 52 degree small angle, length of 377' and width 184'. This might work if you scaled it by 28/430. You could hunt online for the design description and charts of lengths and angles. Remember the feed impedance is 800 ohms so you need a wideband UHF balun and impedance transformer. Without it you'd have an 16:1 swr, and even with a half wave coax balun you'd have a 4:1 swr! It's not impossible but not simple. $\endgroup$ – tomnexus Jul 3 '15 at 5:13

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.