Post by ogrevorbis on Jan 28, 2014 18:42:00 GMT -6
I was thinking about getting a 160 meter ham dipole with 4:1 balun and adding 26 feet (that's what I calculated should work) to it to use on 1700khz. Do you think this will be effective? I want to do AM, but a vertical with groundplane is too large.
Should be good for skywave broadcasting at night but will not work very well for daytime groundwave. If you plan on shooting skip at night then nothing beats the dipole or inverted L once the sun starts going down. For daytime broadcasting it is best to stick with a shortened vertical using a capacitive top hat like the Marconi T. Skip tends to work best with mostly horizontal polarization at night because of the takeoff angle whereas during the daytime you do not want all your power going to the clouds. If you have the power though a shortened vertical can work wonders for local broadcasting during night hours, but we are talking hundreds or thousands of watts.
Example: 10 watts can work a whole town on a short vertical during the day but will hardly get down the road at night when the skip rolls in. OTOH the same power will do squat on a dipole during the day but can work skip at night. At least in my experience anyways.
I take it you are using ladder line at the dipole feed point since you are using a 4:1 balun?
Post by ogrevorbis on Jan 29, 2014 18:19:40 GMT -6
I haven't actually done anything yet. I'm waiting for it to get warmer outside. I was planning on using a 4:1 balun with coax. Is that wrong? There are two reasons why I chose this idea. The main one was because I tried a shortened vertical about 10 feet a few months ago and could not get it to match the frequency even after calculating the coil. Before that I tried a simple end fed long wire (cut to the right length) with a T network tuner attached and I could get it to match, but it would only take a few watts before the SWR went too high. The signal had a range of under a mile and I think I discovered why; the wire going to the grounding rod was getting warm, so I think all my power was going into the ground. I was thinking if I use a dipole, then I don't need a ground, so all the power will go to the antenna (even if most of it is heat).
Your discussion about the MF aerial, viz. on whether it's better a monopole or inv. L type, reinforced my intention to post a message with some issues on the subject, but I'll be dealing with those later in another message here.
Meanwhile, and because there's a Danish member I am corresponding with who actually uses a shortened dipole and is happy with it, especially after having completed his AMU-ant. matching unit, I think that, yes, the dipole may be a good option, but to add something that Kage said, the lower it is, the steeper the take off angle, so not appropriate for local coverage.
However, this "vertical" (the commas are needed for my "monopole" is a slanted wire that's bent at a point away from the house, and the the wire continues [8 m or so] to the radioroom) I am using for months together with the RFsource exciter + amp. was able to put a [vy. poor] signal across the border some 250 km away from me, and what's more is that it was even detected on an SDR in Finland.
My advice is: if space and conditions allow it, then use some vertical *and* a shortened dipole depending on the target you're aiming at.
Again, more my aerial and my AMU and inductor in a separate message, maybe tomorrow or during the next weekend.
Hmm I am not exactly sure why you would use a 4:1 balun. I always used a 1:1 current balun for dipoles at the feed point. At such a low frequency a 1:1 balun would be on the large side so weight could be an issue but with that long of a dipole I am sure you would give it support in the center anyways.
I always thought 4:1 baluns were used to transition between ladder line coming down from the dipole to coax going to the transmitter. I could be wrong though since I have never used a 4:1 for anything related to this. 4:1 would convert a 50ohm unbalanced coax to around 200+ ohms balanced which is a close match for 300ohm ladder line. Most dipoles have a much lower impedance. Maybe the impedance rises at such low frequencies? I have never tried a full dipole on medium wave yet.
Tuning up a shortened vertical on MW is not that terribly hard but becomes more of a hassle the shorter it becomes. Usually around 50 feet or longer it becomes incredibly easy to tune up using a roller inductor and variable capacitor in the configuration shown here.. www.maxmcarter.com/classexmtr/circuitdescription_files/antenna.gif
You simply tune up for maximum current or field strength. Using an ammeter or a lightbulb between the vertical and tuner and tune up for maximum brightness, then short out the bulb. Kind of an old trick but quick way to test/tuneup. Also don't forget vertical antennas do not have to be perfectly straight, they can be bent into L configurations and other weird patterns. The idea is to just get the thing resonating. The more wire up in the air even if it isn't perfectly vertical the less it stops acting like a dummyload lol.
What it boils down to though is if you want nighttime skip coverage or just daytime coverage. Inverted L can give the best of both worlds since it is both vertical and horizontally polarized to an extent, making it work well for both skip and local, skywave and groundwave, respectively. Just pay extra attention to DIY amateur radio antennas for 160 meters. Whatever works the best for them will certainly work the best for 1700Kc.