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.
It's a long time since I last said anything about RFsource and this exciter-amp. of theirs, that I have.
Well, I have decided to add another post, but not specifically to talk about them.
Let me divide this post into 2 parts, viz.:
A) I have been using the tx w/ some success, which merely consists of relaying some radio stn I want and be able to pick my signal on the car radio. Vy. simple, so not a "music stn." at all.
In the middle of this, I have been using this inv. L, but there were changes:
Antenna I reduced the 45 m horiz. portion of the inv. L to just 7.5 m, and kept using the same slanted radiator; of course, there were changes in terms of SWR.
Due to that, I lhave added 3 sloping 7.5 m wires in the hope to achieve higher capacity, and it must have for I experienced changes when using this or that tap on the new inductor.
ATU I finally ordered a 2nd 1,000 pF 1200 V variable capac., installed it, and am stil with the L-C-L variant in T configuration although I briefly tried the Pi configuration for a few days' time. For some reason, the Pi type didn't provide the results I was expecing.
Inductor Instead of the variometer, I made a coil, and the whole thing began to work more smoothly. Detais: abt. 80 turns of 1.8 mm diam., unifilar wire over an 80 cm long, 4 cm diam. form (PVC pipe), taps every 5 turns.
B) Audio & Power Supply.
Audio source. Normally, it's the computer "tuned" to a particular stn I want to relay. When the audio cable is inserted, the RFsource exciter indicated a sligh reflected power increase, then I adjust the ATU according.
Higher audio peaks cause variable SWR/reflected RF. I am still to build an audio limiter as some advised me to do.
However, if I use the JRC NRD-545DSP rx as audio source, something is happening that wasn't when I had the antenna in its primitive shape: the refflected RF warning kicks in and the tx switches to safety mode.
Even connecting the audio cable to the rx causes higher SWR, let alone increasing AF gain.
I don't know how to cure this.
Power Supply. I have ordered a variable V & I model, but it was returned, so I feel it's fair if I say don't used them as they can be affected by RF and "go crazy".
It replacement arrived yesterday - Mean Well for 27 V nearly 6 A. Another disaster! It delivers 30 V, if adjusted, and it's set for 28 V, but when I use it, the max. tx power decreases almost by half and it causes high SWR.
So, the 24 V 6.xx A PS from the broken TV set is what I am still using until I get a solution for the Mean Well model *or* then have a custom made PS built for me. _________________
So a few words of advice: stay clear of RFsource, which I hate to say, and avoid those PS types, get a linear one, even if this means having it built for you, if there isn't anything on the market with the needed tension/current.
More could be said about the antenna & inductor, but this is already a too long speech.
Folded monopoles (I am not sure whether this = cage antenna, but fear not) are often used by b/casters whenever they want to use the tower for their VHF-FM antennae on top thus saving costs for a separate tower exclusively for those VHF antennae.
This is not entirely impossible with ordinary monopoles, also known as plain verticals, but refined requirements are needed.
As for a ground system, both folded monopoles and plain monopoles *need* good grounding, typically in the form of 120 radials scattered around the base.
I have seen folded monopoles on top of insulators, but I'd say the typical ones don't need those.
In here www.qsl.net/aa3rl/ant2.html, one can see that several comparisons taking 7 MHz as an example, but this can be extrapolated to, say 1650 kHz, meaning a small fraction of this wavelength demands a very tall dipole. Of course, if this no problem, and also if one wishes to beam the signal on two opposite directions, then maybe the dipole for 1.6~1.7 MHz is not too unpractical.
Maybe I switch from inv. L "mode" to T "mode", at least for an experiment, but unfortunately the 2 "arms" of the T can't be horizontal, they must be installed in a slant manner due to space restrictions.
However, I could perhaps make a lower T and then have its arms horizontal or nearly horizontal.
Would you recommend me to try it, or do you think it's a complete waste of time & effort, if the vertical leg of the T is no longer or taller than, say, 10 m or a bit less?
One more item in this post: what about a [necessarily short for obvious reasons] folded monopole? I wonder whether any pir. stns. ever tried this, or if so whether some are actually using this sort of vertical.
Well, I usually call it "observations" while others prefer the term "loggings", and if you check the [Danish] URL I mentioned, then you know what sort of stations I "deal with". While in the right section of that page, viz. "Loggings", my obs./logs are identified by the initials "CG" or "CGS".
From your words, Kage, I take it BC stations are excluded, and that's understandable as the name of the Forum speaks for itself, but I wanted to ask so as to know for sure.
If space for an antenna is an issue, or if, as you say, local coverage is your goal (well, it also depends on "how" local you mean!), then note a MF antenna may be quite short indeed. Even LF ones can, all depending on the compensation they have in order to be able to work on the freq. they're cut for.
Of course, what you see on the photo is not only short but also compliant with actual radio regulations, viz. those of the USA.
If the latter part of my sentence is not an issue, just consider the physical aspect of the monopole. If space is not a problem, then you should use it.
There was one "tiny" mistake in the description of my current inv. V - maybe you noticed -, so I have just corrected my text accordingly.
The main band for this antenna is 60 m, not 31 m as I put it. The problem whn using similar aerials for harmonic fqs is the amount of lobes one gets meaning this one in particular has a good array of them on 31 m.
As for MF, I wonder what sort of aerials many of the MF pir. stns I can get actually use: inv. L or T? Perhaps as they're the most common, but possibly plain verticals, like the huge majority of the b/casters you mentioned.
Speaking of them, this page is particularly interesting and useful: waniewski.de/id208.htm
"Thousands of watt". Well, even a 1 kW MF b/caster can be heard thousands of km away. I don't mention less than 1 kW for practical reasons, but in Europe alone, the several hundred watt stns of b/casters like Gold, Absolute R and a few others incl. BBC locals from GB are regularly audible by me. One of the funniest things recently is this stn I keep receiving on 1386; lists indicate the British stns using this ch. are all 1 watt! No ID so far.
Also, trans-Atlantic reception of MF stns using 1 kW or less is possible, and if you go to the NDBs on either LF or MF, then power is certainly in the order of a few hundred watt only. Now that I mention NDBs, one of the latest casual catches was this Brazilian oild rig NDB off São Paulo state, Brazil, 1645 kHz, call MLZ, obs'ed. several times after 11 Aug around 2200 UTC. They do have a few operating above 1610.
I forgot to add yet another word or two about another sort of antenna you referred - the dipole. Needless to say it demands a lot of space, *but*, even if one has it, what about the height above the ground?
Installing a dipole may be "easy", if one only considers the space, but if a horiz. dipole is too low above the ground relatively to the wavelength is's supposed to work on, then the antenna behaviour is highly disturbed.
A low dipole erected too low will send the signal on a too steep angle causing the signal to be useless on short / medium distance. Conversely, it may receive signals that arrive on vy. high angles.
I had a similar experience for years on end with this inv. V formerly consisting of two portions of 21 m; a few months ago, I shortened it to just two portions of 15 m, and lowered it a great deal. This is fed with 450 Ohm "ladder" line into a home made transformer inside the house (which is in many cases not a good idea, even if the antenna is used for rx only, which is the case). So the current dimension suits the 60 m<>5 MHz band.
The result after shortening the wire and lowering the whole thing was evident. Reception from the beams it's serving (SAm / Asia) improved a lot. Of course, it also serves the 31 m<>9 MHz band in particular.
To be honest, I don't really like to refer to "inverted L", but I did because this name is more widely known.
To Jowijo:
The issue here is that a T and an inv. L are almost the same: while the latter has "one leg" acting as "top hat", the former has an even portion of wire serving the same purpose. Also, Kage said it all - "playing" on MF is *not* the same as on VHF-FM where it's f a r easier to make an antenna, be it omnidirectional or directional.
Back to Kage... to say or rather to ask whether your comment about the inv. L having a bit more horiz. polariz. resides on the fact that the "top hat" does that when it's a long one.
I suspect it does as mine always exhibited some directivity "à la" Beverage on HF fqs like the 6 MHz<>49 m.
Well, back to my evening's DX session: I finally heard the ID of the Dutch stn on 1633 - R.Barcelona, now with an improved signal.
There are other far more technically minded members in this Forum who are able to help, but my own experience as an inv. L or Marconi user makes me suggest you can also use one.
If you can, use a vertical, the longer the better, and add a top load, "top hat", and, of course a tuning unit of some sort.
I am using a C-L-C type antenna tuning unit together with a variometer, and the antenna itself consists of a 20 m slant wire (I couldn't get those 20 m entirely vertical)connected to a [long, vy. long] "top hat", 45 m (!).
No, you are quite correct, MF stands for medium freq. <> M.Wave. I rarely use abbreviations such as LW, MW, SW; I prefer the ones referring to freq. rather than wavelength.
And if there's something I particularly hate is the use of the terms "AM" & "FM" being applied to freqs. or waves.
The antenna you depicted is a monopole, and I forgot to add, that if one varies the length of the radiator, the take off angle (and conversely the receiving signal angle) will vary too.
If too short a radiator, the coverage will also be reduced.
This happens a lot with MF broadcasters using this sort of antenna, but the "radial" system they use is at a 90º angle relative to the radiatior, and are burried in the ground, typically 120 of them, and almost as long as the vertical element itself.
For instance, a 1 kW tx fed to a half wave monopole will have a larger coverage than the same tx fed to a 1/4 wavelength monopole.
Is it pratical, I mean using a Delta Loop for the 87.5-108 band?
It surely resembles the K9AY, which can be fed up or down depending on how one erects it. In my two cases, both [elevated] K9AYs are bottom fed, triangular shaped, each using 4 loops, 25 m perimeter. These are my main tools for LF/MF DXing. One of the K9AY has 2 loops with 2x25 m in series so as to increase sensitivity.
The ones below are those I mentioned, plus one other I tried and still have, but it's just standing there, not used at all.