Dipole a little deaf

[VK5IR]

Hi all

My antenna knowledge is not the best (as your about to find out) so please don't flame me.

I have a home brew half wave 40m dipole centre-fed with choke balun up around 6 meters high. The length of wire is ROUGHLY 10 meters on either side.

The antenna works very well on 40m, it also gets out on 15m, not too sure about 80m (but at this point I am only interested with 40 & 20 meters as far as this antenna in concerned). Its not resonant on any of those bands, but my MFJ tuner does a very good job of tuning it up.

It is however, a little deaf (I think) on 20m as most of the stations I hear on 20 are pretty low down in the noise. Perhaps the conditions have just been crap on 20, I dunno... The noise level on this antenna is around S2-S3. Switching over to the station master brings the noise up to S7 but its just noise.

Now, I thought being a full wave length long on 20m would have made it a pretty good performer for the 20m band. Or am I wrong?

Many thanks for your help

Theo.
VK5MTM.

[VK4ADC]

Theo

The problem is that the antenna might be a half-wave at around 40m, and indeed a full wavelength at 20m but the real issue is that the feed impedance at the centre of a conventional half-wave dipole is low (about 60-70 ohms) while the centre of a full wave dipole is quite high (hundreds to thousands of ohms). The low impedance feeder (coax) works well into the half wave dipole because the impedances approximately "match". A tuner may "synthesise a match" as far as the radio is concerned but the actual efficiency of an antenna operated away from true resonance is poor - and that goes for 40m operation too. Spend a bit of time tuning the lengths so that it resonates around 7080 to 7090 and signals even on that band will improve.

The only way to get a low SWR on 20m as well is to add an extra dipole of about 5m each side to the same feed point as the existing 40m one, and spread away from the original wires. The antenna then becomes a "ray" dipole and will operate on 40, 20 and (to a degree) 15 too. It won't work on 80m or 10m because of the harmonic relationships of those bands to the fundamental resonances of the (now) two dipoles. A frequency must be an odd multiple of the basic half wavelength to provide a low impedance at the centre eg a 7MHz basic dipole gives 21 MHz and 35 MHz (but that isn't an amateur band ) possibilities. A 20m basic dipole's 3rd harmonic is 42 MHz but that isn't an amateur band either.

You should also be aware that an antenna operated at a harmonic will typically "resonate" lower than the direct multiple of the original band's resonance. For example, a dipole resonating at 7.080 typically provides the lowest SWR at around 20.9 - 21.0 MHz (rather than the expected 21.24 MHz { 7.080 x 3} ).

Visit my web site to see a bit more detail on multiband-ing HF antennas : http://www.vk4adc.com

[VK3YE]

Theo,

I agree with Doug's comments.

The basic trade-off with dipoles and feeders is multiband capability vs convenient feedlines and avoidance of tuning units.

Coax is handy, plugs straight into the rig but makes the dipole essentially a single band antenna (except for the 7/21 MHz combo mentioned). Adding a parallel dipole is a good way to get 20m as pointed out. But when you add more bands it looks ugly and there's a risk of interaction. Have a look at VK5AH's dipole at http://users.picknowl.com.au/~wavetel/antennas.htm I've had good results with adding end loading coils to a 40m dipole to make a usable antenna on 80m. Just two wires should give you 80, 40, 20 and 15m (may need to tune the latter with an ATU but loss shouldn't be too severe).

The other alternative is open wire feedline. This makes the antenna multiband but you'll need a balanced line antenna coupler. It still won't be that good on 80m - an effective dipole needs to be at least 3/8 wavelength on the lowest frequency - much like the G5RV.

Check out VK1OD's website for comparisons with regards to losses and multiband dipoles. Some popular designs don't stack up too well. If you can afford it an auto tuner at the bottom of a tuned feeder dipole is pretty efficient. Or make your own tuner.

[VK3AIF]

Hi Theo,

Have a look at the W3DZZ antenna, mine works fine on 80, 40, 20, 15 and 10 to a degree. 10 has two dips in SWR at around 28.3 and 29.4 from memory but is a little higher than I would like at 28.5 not that I have been inclined to use 10 much lately but it is picking up. It is configured as an inverted "V" with the apex at around 10 meters wit the end at about 3 M AGL. One of the big problems with antenna on the lower part of HF is the height in terms of wavelength is small and this causes the feed impedance to also be lower than desired. A good SWR on a low horizontal antenna to me signifies high ground losses.

Height is might on the longer wavelengths.

[VK4WDM]

After over 40 years playing with wire antennas I have a very simple saying: "the simpler the better." Avoid traps if you can, they are lossy.

Open wire feeder (300 ohm ribbon is ok) does work better than coax for multiband dipoles, and although it will make the purists shudder, you can use a 4:1 balun and a short length of coax to connect it to the rig if you don't have a balanced coupler (which you can build).

Despite what the "G5RV haters" say, it does work well as a multiband antenna. I worked DXCC in two years with one. If you don't have the room for a full sized one a half-size will get you 40-6m.

Having said that, if you really do want to optimise for 20m just put up a single coax fed dipole carefully cut and tuned.

The other thing to consider is that now is not the time to be to despondent about "deaf" antennas - the bands are lousy and even the guys with big beams are have trouble getting contacts.

Finally, be warned. If you play around with wire antennas too much you will become a TAWAN - a terrible thing to happen!

73

Wayne VK4WDM

[VK3ZFS]

At my remote bush block QTH, im using a full wave 80mtr terminated delta loop, using 16:1 balun and 800ohm non inductive termination, its SWR is under 1.8:1 from 1.8 to 30Mhz, it also makes a fantastic RX antenna.
I know I loose a bit of power with the balun and termination, but not using a tuner, I gain some advantage

[VK5IR]

Thank you all for your helpful replies.

I am not currently in the position to put up multiple monoband dipoles nor can I use open wire feedlines with my current setup.

add an extra dipole of about 5m each side to the same feed point as the existing 40m one, and spread away from the original wires. The antenna then becomes a "ray" dipole and will operate on 40, 20 and (to a degree)

I am seriously considering giving this ago and seeing how it performs. Is this what you also call a parallel/fan dipole?

Basically I am thinking of running roughly 5 meters of wire either side of the choke balun in parallel to the current 40 meter half-wave dipole. I'll be using PVC spacers (unless someone can suggest a better material), but I'm just not sure how far apart the two wires need to be.

I hope I am making some sense here. Kind of like whats in this diagram http://www.google.com.au/imgres?q=fan+d ... 29,r:5,s:0 but with only two wires either side:

Yes I understand this is not the best way to go about things and that there are probably a hundred other types of antennas/setups I could try but this is the most convenient for me now.

Many thanks again

Theo.

[VK5ZD]

Hi

You could also try an off-centre feed. If the feed point is at the 1/3 point (i.e. 3.33m from one end of a 10m wire) it should work reasonably well on 40m, 20m and 10m (but not 15m). You'll need to use a 4:1balun as the feed impedance will be a couple of hundred ohms or so at this point.

73
Iain

[VK4ADC]

Is this what you also call a parallel/fan dipole?

The multi-wire dipole has many names including the fan, parallel, ray and probably a few others.

The important thing is that you need to carefully adjust each side so that they are the same length - and - also adjust the lengths to resonate at the desired centre-point/target within the band. I suggest 14180-14190 for 20m and 7080-7090 for 40m. Just throwing 5m of wire on each side is not an option - it has to be properly adjusted.

I used spacers made out strips of acrylic from an old car window weather shield for quite a while but almost anything that won't crumble under Australia's UV, or is very RF lossy, is suitable.

There is a page on my website that shows how much length versus frequency change occurs for each HF band : http://www.vk4adc.com/content/index.php ... &Itemid=45. If you measure your dipole's existing resonant frequency, you can do a quick determination as to how much to shorten/lengthen it to move it to the target frequency.

I have tried OCF's and they do work but tend to sag badly because of the weight of the balun and feeder if these are not fitted at the main mounting point with the two wire sections leading away to end mounting poles.

Keep at it, it will work for you.

[ZL4TAE]

Hi Theo
Try a pair of coax traps refer here for some good info
http://members.shaw.ca./ve6yp/index.html

Cheers Terry ZL4TAE

[VK5ZD]

Try this link instead: http://www.qsl.net/ve6yp/index.html

Iain