Just wondered if I could pick your brains on some antenna theory.
Will a loading coil have the same effect on a end-fed ½ wave (vertical) as it does on a ¼ wave vertical (ie allowing you to shorten the vertical radiating element)?
My gut feeling is that it probably won’t work because an EFHW is already fed at a very high impedance (hence the need for the 49:1 transformer to feed it), so adding more resistance in the form of inductance probably isn’t going to work?
Suffice to say that my antenna theory is very sketchy at best, so I might have this completely wrong!
I’ve currently got an end-fed ½ wave for 10m built on a fibreglass mast which is attached to a aluminium pole mounted on the side of the house. The base of the fibreglass mast is at about 7-8m above the ground which gets it up above all of the local “clutter”. It works great on 10m (the local club hosts an FM net on 10m), but other than that 10m is pretty dead.
I’ve previously tried to get 17m from it by extending the vertical element to the full length of the fibreglass mast but the mast is just a little bit too short. It ends up resonating at around 18.6MHz.
I wondered if I might be able to load it up on 17m (or maybe even 20m) using a loading coil in the same way you can a ¼ wave for 80m or 160m?
I don’t really want to go to the effort of getting the ladder up the side of the house, pulling it all down to reconfigure it then put it back up again if I’ve got the theory wrong & it’s not going to work!
If you think of what an ATU (antenna matching unit) does, to make a short antenna work on a lower frequency it adds inductance or capacitance to match the length. Now this isn’t really "tuning the antenna as the antenna doesn’t change length and hence is not “tuned” per-se.
Looking at it another way - leave the inductance and matching issues to the side for a moment - by adding a coil at some point in the wire, you are physically extending the length of the antenna to be what is required for e.g. 17m. This is actually antenna tuning - but you rightly realise that by adding the coil, the impedance seen by the UNUN will also change.
The best way to work this out is to use an Antenna Analyser but first of all you have to make a decision - do you want this to be a multi-band antenna or simply change it for a different band. I am presuming the second option - to do that you need to extend the length, which you can do by adding a coil but a better way is to extent the length of the wire and at the top of the mast bring it back down at an angle and tie it off with insulated cord (this is what could be called an Inverted-L antenna) - the closer to the horizontal you get the extra bit, the better - perhaps tie it off to the house guttering (after the insulated cord).
If you want this to be a multi-band end-fed wire, then you need to chose a length of wire that is NOT resonant on any amateur bands and you “tune” the antenna with an ATU.
P.S. what are you using as a counterpoise wire - not the coax shield I hope!
I know several commercial suppliers who state “no counterpoise needed” on their EFHWs and at QRP it’s no so bad but apart from the RF going where it shouldn’t the other point is that the counterpoise is the other half of the antenna and hence when using the coax feed as the counterpoise, it’s length and what it is close to will affect the tuning of the antenna.
I know it is not 100% true, but think of the driven element wire and the counterpoise as being like an off-centre-fed dipole and you’ll see how important the counterpoise or radials are. Even though this means half of the antenna is on the ground, the piece in the air has something to work against.
If you can add a separate counter-poise wire to the antenna, I would expect the signal reports that you receive will improve. This won’t solve your 17m band problem but it’ll put the complete antenna system in a better light. If you are limited on space to run out a counterpoise or radials, take a look at the “Loop on the ground” counterpoise system featured in RadCom a few months ago. For a small back yard that can be a good counterpoise for a vertical antenna.
The “Loop On Ground Counterpoise”. You don’t hear about that very often! It seems to be one that is often overlooked.
I have experimented with this and it worked much better than I expected on a 1/4 wave vertical for 40m.
I also managed to load it up on both 80m & 160m with a loading coil with reasonable results (considering the short length of the antenna & comparatively small amount of wire in the ground in the loop counterpoise system).
The difficulty with that counterpoise in this particular setup is that this antenna is on top of the mast way up in the air (well above the roof).
I could run a counterpoise wire away from the mast at an angle to the bottom of the garden & choke the coax at the base of the antenna. A lot of literature seems to suggest that you only need 0.05 wavelength counterpoise for an EFHW (just 1 meter on the 20m band), so most of that run down the garden would be guy rope! Might be worth it though if you think that this will yield a significant improvement?
I wanted to get the antenna up as close to 1/2 wavelength above the ground as possible to get it above the roofline and get as low a take-off as possible (I seem to recall reading somewhere that 1/2 wave above the ground was apparently the optimum for an EFHW).
There are lots of other buildings around my house, so mounting on the ground surrounded by other buildings isn’t really optimum. Hence the reason I mounted the fibreglass mast with the antenna on top of a 6m aluminium mast.
Ground mounted is OK for the lower bands, say 40m & below. On the higher bands I think it would be better to get the antenna in the air above the surrounding buildings.
You have the luxury of having the vertical high-up then!
How about this for an idea - rather than adding a coil at the base of the vertical, as you are high up, perhaps you can come out horizontally from where the UNUN presently is with an insulated wire to make up the missing length - if the vertical is currently a half wave of 10m, that means it goes up 5 metres if you were able to go out horizontally where the metal and fibreglass masts meet by 3.5 metres and then re-attach the UNUN, you would have an EFHW L-Antenna for 17m. This would still be a single band antenna though, but you could (if you don’t mind going up the ladder to do it) have connection points on the horizontal section to reconnect the UNUN to at 1m from the masts (12m), 2.5m from the mast (15m), 3.5m (17m) and 5m (20m).
This is assuming you have this distance available horizontally outward from where the UNUN is currently.
If the UNUN point is not at say the level of the (hopefully plastic) gutter, this could be more difficult but the “horizontal extension” could come down at an angle to the point(s) where the UNUN will sit.
Just an idea - adding a coil will work but extending the length of the antenna will be a better solution even if the resultant antenna is part horizontal, part vertical.
Just one simple question: If I design coil loaded (shortened) 1/4 wave antenna for say, 900 kHz, using 18 m Spiderbeam fibreglass pole and wire, will it resonate as 1/2 wave end fed antenna at 1800 kHz? This is my idea how to get vertical portable antenna for 160 m without extensive radial system using tons of wire…
Top Band used to be the mobile band of choice back in the days when you had to apply for a /M licence. I remember at rallies the car parks were a forest of long antennas with huge coils sprouting from rear bumpers. The losses were probably enormous, but I often heard evening contacts /M into the near continent on my old R1155N receiver! IIRC the power limit at the time was ten watts.
The next step from loading coils would be to explore a helically wound vertical for the lower bands. ISTR remember such an antenna being described in Radcom some years ago.
I agree that looses will be high, but how high in comparison with 1/4 wave top band or, inverted L, both require large radial system. And yes, i plan to helically wound wire around the pole, then measure with antenna analyzer where i am…Probablly, i should add some more inductance to get where I want. I already made matching unit, this is link coupled air wounded coil with variable motor driven capacitor remote controlled. It works great with all 1/2 and multiplewave
To work on matching a very short aerial we often use j notation. So resonance in a 50Ohm system would be represented by:
A very short vertical could be 2-j2000Ohms.
We need to match two issues:
2 ohms to 50 Ohms resistive, this can be matched with a pi network.
2000Ohms reactive to resonance. This can be matched with a inductor that has a reactance of 2000Ohms in series with the feed.
Finally we need a ground plane to minimise the losses.
Many authors refer to the efficiency of such an aerial being only a few percent.
So if we feed the aerial with 100 Watts only 2 or 3 will be radiated. The reception will also be a few percent effective.
A few years ago I had a car with a home brew centre loaded aerial on the boot. With different coils it would transmit on 80, 40, 30, 20m. The matching was by pi network and resonance achieved by adjusting the length of the pole above the coil. So it can be done, but don’t expect miracles.
I would imagine if you only want to drop the resonate frequency from 18.6 down to 18.1 say in the middle of the 17m band you would not need a very big coil just a few turns of wire. Don’t see it making much difference to the feed point end at all. How would you get on if you added the required wire to get a lower resonate point and turned it into a sloper away from the pole to make up the lenght or make the pole slightly longer if you want to run the wire close to said pole support.
The old amateur spirit try it and see.
G4AZS, thank You for your post. My first goal is to avoid extensive radial system for 160 m. I still believe that i could design such an antenna, I don’t see reasons for dramatically high losses, because it would be END fed antenna with relative high impedance at feedpoint, so ground losses should not present problems as it is case with shortened quarter wave antennas. Second, that type of (oh, yes, full size 1/2 wave) antenna require only 0.05 wavelenght counterpoise. I could install 2 or 3, not 100+ long radials. For sure, I will try. Just like to know what others think about it, maybe someone already tried that?
There are enough endfeds that can be extended to lower bands using an loading coil. (e.g. endfed for 10/15/20/40m with a coil for 80m)
Why shouldn’t this also work for 160m. The question is the position and dimensioning of the coil.
Of course, a 0.05 lambda pigtail or a dimensioning of the coaxial cable (0.05 lambda shield to the current balun) might be helpful.
If the vertical is installed on a 12m mast, the length and therefore the effectiveness is perhaps even better than the 1.4m telescope of the Elecraft AX1 at 20m … sure better than for 40m.