The math says my setup is decent, but I think otherwise

Discussion in 'Ham Radio Discussions' started by N6AVI, May 2, 2021.

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  1. N6AVI

    N6AVI Ham Member QRZ Page

    As an engineering student, I was taught that the calculated numbers are usually 99% right. Which brings me to by current dilemma: my ham setup.

    Having spent some time reading over the basics of certain theories and calculations in amateur radio, this is what I concluded:

    I currently have an Icom IC-735 that outputs 100W. My setup is a homemade dipole for 10m (antenna image on my qrz profile), with exactly 136 ft of RG-58 coax. The feed runs over my roof from the front of the house (where the shack is) to get to the backyard where it's currently mounted. At the moment, the dipole is held by a (last I remember) 13.5 ft PVC mast, slightly below halfwave, though that will be rectified soon, but I'm going to discuss with what I have now regardless.

    Looking at the calculations this is what I have concluded:
    Current Output with 136 ft of RG-58 [using dB = 10log(P2/P1)] = 47.1 W (Ugh)
    Elevation Angle at 13.5 ft [using theta = sin^-1(o.25/h)] = 24.3 degrees (I don't think this is terrible??)

    I have read that using a different coax like RG-8X can help significantly, but running the numbers still gets me to values such as 57.9 W for output, which I feel is still somewhat degrading.

    In regards to antenna height, even if should I get it to at least halfwave high, it will only get me to 30 degrees of takeoff elevation, not too far from 24.3.

    I guess what I'm really trying to ask from here is if this is the best I can do with my current parameters, or if these slight differences in values actually make a bigger difference than I am realizing. I understand the higher the power the better, and likewise with antenna height. But in terms of the elevation angles, as far as I know, a lower take off angle means a better chance to propagate further. So if by adding height from what I am seeing actually increases the angle, doesn't that mean that the range decrease at this point? This is where I'm contemplative about the numbers here.

    I was thinking maybe getting more components like a linear amplifier might help here, but there might be something here that I'm not seeing that would likely tell me not to go in that direction yet.

    And for those wondering, this antenna isn't grounded. Still being a relatively new ham, there are a few topics I have not yet ventured, and grounding antenna systems is one that I still have some trouble understanding, sorry to say.

    I've exhausted options here up to this point. I just really want to be heard as I hear people over the air here during openings and whatnot. Anyone that can provide some input on this I would be greatly appreciative of. Thanks for reading.

    -73s, Mat [N6AVI]
  2. KA0HCP

    KA0HCP XML Subscriber QRZ Page

    Hi Mat, welcome!

    Without modeling, a dipole (any dipole) located about 1/2 wl high will have an essentially omnidirectional pattern and will have much of its radiation at covering high angles.
    Here is my favorite engine for doing rough estimates on system losses.

    For RG-58A, 136ft long, 1.2 load SWR, you will have about 49 Watts reaching the antenna, due to coax losses.

    The best things you can do to improve your 10m antenna are:
    -Elevate as high as possible. The classic Figure 8 directional pattern and lower take off angle will develop as height approaches and exceeds 1 w.l.
    -Shorten your coax length.
    -Use lower loss coax.

    -Keep in mind that when 10m opens, it tends to be very good propagation and power is less important than on lower bands. As a reminder, with solar flux and sunspots still low, 10m openings are occasional, rather than daily and reliable; that is part of the fun of hamming.

    Enjoy! bill
    N8ZL, N4OKN, PU2OZT and 2 others like this.
  3. KP4SX

    KP4SX Premium Subscriber QRZ Page

    A half-wave high antenna is a bare minimum. It will suck!
    135ft of coax. Ufff. I'd recommend some good stuff even better than RG-8X.
    Amplifier doesn't help reception.
    Put away the calculator and get the antenna up!
    WA7VBA, KF5ONT, W9JEF and 2 others like this.
  4. KL7AJ

    KL7AJ Ham Member QRZ Page

    47 watts is more than enough power on 10 meters, when the band is open. When conditions are like they are now, a gigawatt won't even work. Your line loss calculations are about right. You'lll get some improvement going to RG-8 coax, and a LOT of improvement going to window line or ladder line. In any case, you'll only gain a fraction of an S-unit. But for now, 10 meters sucketh mightily, and you can't do much about it. :(
    N8ZL, WA8FOZ, K0UO and 2 others like this.
  5. N1EN

    N1EN Premium Subscriber QRZ Page

    I'll offer two thoughts:

    1. While it's good to do the best with what you have to work with...analysis paralysis can be a risk. Put it up, and see how it works for you (being mindful that 10m is tough at this part of the solar cycle). A few degrees of peak radiation angle won't make that much of a difference, and real-world considerations (like soil conductivity and ground reflection therefrom) don't always translate well into the models/calculations.

    2. Not withstanding the above...RG-58 is very, very lossy. I wouldn't recommend using it, especially for 10m or above, if better options were available. You've already picked up on this with your calculations. With that long a run, even RG-8X is not wonderful. LMR-240 has been my go-to for HF antennas, as it's at a sweet-spot in terms of cost vs performance. RG-213 and LMR-400 would be even less lossy, but LMR-400 starts to get expensive, and both cables are heavy enough that strain at the dipole would be a concern.
    N0TZU, PU2OZT and N6AVI like this.
  6. K0MB

    K0MB Ham Member QRZ Page

    There is NO propagation at 10m without an active ionosphere! (let the debate begin!)
    N8ZL, PU2OZT, K8BZ and 5 others like this.
  7. WA9FZB

    WA9FZB XML Subscriber QRZ Page

    Mat - Is there any reason why you are focused on 10 meters? In the current low-sunspot period, 10 is the most difficult of the HF bands on which to make contacts. The propagation is just too infrequent up there.

    I would recommend looking for a way to put up an antenna for 40 meters, too. At almost any time of the day, 40 is open to somewhere. I would also recommend that you look into CW operation. I was QRT for some years, and when I came back onto the air, I only had a 40-meter dipole made from two "Hamstick" mobile whips. That dipole was mounted on a 5-foot length of TV antenna mast and stuck into the weighted base for our patio umbrella. It stood on our patio, out behind the house, in a city neighborhood. Over that winter I worked probably 25-30 US states, as well as some DX, using 100 watts on 40 CW. I "almost" never had to hunt for a contact.

    Digital modes are even better for making DX contacts when other modes fail.

    It appears that you will be working with some sort of a compromise antenna right now. If you can figure out a way to get on 40, or even 20 meters, there will be a lot more contacts to be made.

    Welcome to our crazy hobby!
    N8ZL, K8BZ, N6AVI and 1 other person like this.
  8. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    ^This isn't correct. At 4m height a 10m dipole over average ground has a TOA ~27 degrees. At 8m (double the height) that reduces to ~17 degrees, which is better. Raising a horizontal antenna higher above any kind of ground can never increase its takeoff angle.

    But another advantage of "higher is better" is reducing ground loss and getting the antenna above obstructions which include everything around it that isn't air.:p Raising the antenna >1WL will form multiple radiation lobes at different angles, but the primary lobe always becomes lower in angle, even while new lobes form above that. Simple mathematics don't predict this. Simulation programs (modeling) does, but so can simple field strength readings.

    The radiation angle cannot be higher with a higher antenna; but it also cannot be calculated using such simple math. Even soil conductivity below and around the antenna for several wavelengths changes all of this.

    10m is experiencing some ionospheric propagation lately, but as far as I can tell, it's sporadic-E and not F2-layer stuff which requires more ionization (check the SFI...for 10m to experience the "big" F2 openings, SFI would normally need to be >100, and the higher, the better: It hasn't been up there yet). But I've worked coast-to-coast a few times on 10m lately via what is probably double-hop Es, which can be short-lives so you "just have to be there when it happens" and isn't terribly predictable. Stations in Florida have been rolling in now and then, even this week. (I'm also in Los Angeles, but using a beam at 55 feet.)
    This is good data: If you hear them, but they don't hear need more transmitting power. While "most" stations may be running 100W output power, lots of hams are running much more than that, up to 1500W output power, which is a rather huge difference (about 12 dB). 12 dB is a lot, and can be the difference between "Wow, I hear you great!" vs. "I can't hear you...try again?"
    N0TZU likes this.
  9. N6AVI

    N6AVI Ham Member QRZ Page

    So then would it be right after adjusting antenna height to get an amplifier?

    I've been noticing a lot of unusual activity around lately too. Solar winds and recent sunspot activity seem to be what is driving it. Received Australia on 10m last night along with other pacific areas, even seeing 17m active from Fiji up until late last night, as well as WWVH being S9+ all the way up to 1 AM weirdly enough. Just now I saw an FMBC opening within the last hour, along with a 2m CW beacon report from FL to KS. It's only May, and it looks like it might be an interesting season this time around, which is why I want to be ready with a sure setup.
  10. N6AVI

    N6AVI Ham Member QRZ Page

    Mainly for space reasons I haven't been able to make anything bigger in terms of antennas, so i'm stuck on 10 for now. I was thinking of making a wire antenna on 20 into an inverted V from my current mast for the 10m dipole, but unsure how that would work.

    Thank you. :)

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