MARTIN - G8JNJ

I’ve recently been looking at designs for an efficient antenna that would fit in a loft. I hoped to find something that would work on with a 100 Watt HF transceiver on all bands from 80m through to 6m.

Searching the web and emailing other Amateurs who use loft based or stealth antennas provided lots of advice which provided a very good starting point.

I have an area about 12m long by 4m wide and 1.8m high to play with, so the first suggestions were an open loop running around the outside of the loft, a fan dipole with wires cut to resonate on each of the required bands but trap loaded on the LF bands and a Zigzag end loaded dipole. At this stage I was considering just using resonant antennas, but it soon became apparent that I would only obtain very limited bandwidth operation on 80m, so the use of some form of ATU became inevitable.

Modelling each of these in turn with EZNEC demonstrated an equal number of advantages and disadvantages for each design. They all gave about the same peak gain at about the same elevation angles, the peaks and troughs in the azimuth plots varying slightly with frequency and wire positions.

One I had narrowed the options down, I installed some wire and started testing.

The first antenna was a simple Zigzag dipole which I fastened to the roof spars using electric fence insulators. Previous experiences with indoor antennas had demonstrated to me that it’s a good idea not to fasten antenna wires directly to any structure. The efficiency improves with 30cm or more spacing away from masonry or woodwork, but even a few inches makes some difference. I think this is due to some sort of close field dielectric coupling, but I’ve not investigated the effect fully. A further reason for using the fence insulators was to keep high voltage nodes on the antenna away from potentially flammable material, as I didn’t want any arcing to create a fire risk.

Here’s the first attempt at a design.

This seemed to work reasonably well on 40m and above. However it struck me that I could add some sloping end loading wires to improve the performance on 80m.

This lowered the resonant frequency which improved the performance on 80m slightly; however it still proved a difficult match on some of the HF bands, where it was approaching ½ wave long.

My next attempt was an open loop wrapped around the edge of the loft space.

This was very disappointing. The although it provided a slightly more Omni-directional pattern on the HF bands. The gain was noticeably lower (10dB worse on 80m) in comparison to the previous Zigzag dipole, so it was quickly abandoned. In theory this antenna should have worked much better than it did, I think part of the problem was the close proximity of the wire elements to the structure of the building and the mains wiring.

I next tried building a series of parallel connected resonant dipoles in a fan arrangement. However it soon became apparent that I couldn’t really get sufficient spacing between the individual wires, and their resulting interaction made it very time consuming to get an acceptable match on all the required bands.

Based on the experience I’d gained so far I decide to try modelling a bowtie antenna.

This looked promising, but then I remembered that I’d seen end loaded versions of these and also Biconical antennas being used for EMC testing. I knew they could offer a wide operating bandwidth, but that they had a 200 Ohm feed impedance, however this would be relatively easy to match via a 4:1 balun.

After playing with various EZNEC models, the best arrangement seemed to be like this.

The two up-turned ends fitted nicely into the available space and provide additional end loading.

The modelled SWR when fed with via a 4:1 balun also looked good, and would provide an easy match for the ATU over a substantial frequency range.

As the feed impedance of the antenna never exceed 1Kohm, I decided that it may be more satisfactory to use a 1:1 balun (12 bifilar turns on 4 stacked FT240-61 cores, see next section on page), so that there was less loss when operating into the low resistive component of the antenna input on 80m. I tried adding more wires to see if this made any further improvement, but I felt that it wasn’t worth the additional effort. However I did decide to use thicker wire for the elements in order to reduce the resistive losses on the LF bands. I’m not sure exactly how much of a difference this made, but I had a lot of 75 Ohm satellite coax off-cuts which I put to good use for this purpose.

Here’s the measured SWR of my first attempt fed via a 4:1 current balun.

Discussion on eham elmers forum

Best options so far

(Red Trace) 5 x FT240-31 stacked with eight turns - good very wide bandwidth, mainly resistive impedance, moderate core loss

(Green trace) 4 x FT-240-61 stacked with 14 turns - Narrow bandwidth, very high impedance, low core loss, large number of turns

(Blue trace)  4 x FT240-K stacked with 12 turns - Narrow bandwidth, very high impedance, low core loss, moderate number of turns, high cost

Plots showing common mode impedance

S21 Transmission Gain

Rp (indicating core losses)

Measurements made with AIM 4170B,  TL Gain with miniVNA,  Graphs produced with Zplots

Ongoing investigation

M. Ehrenfried - G8JNJ - V1.2 - 28/10/2009

Following a discussion on eham relating to the use of 4:1 Unun’s for multiband operation in conjunction with 33ft vertical antennas. I referred to measurements I had previously made with 10m vertical antennas fed with 4:1 Ununs. Most of this work was done as part of an investigation into the performance of the Comet CHA-250 broadband antenna, which uses a deliberately lossy 6:1 Unun to provide a good match on all bands.

As part of this investigation I discovered that the Comet antenna actually worked better than I would have expected it to. In some cases it produced similar results than those obtained with a 6.5m vertical fed via a 4:1 Unun at the base of the antenna and fed with coax from an atu-tuner in the shack. Some comparison graphs can be found here.

As you would guess there is a large degree of interaction between the Unun and the impedance presented to it, especially when it is being used as a multiband antenna.

The two main problems are major copper losses when feeding a vertical which is electrically too short (due to the very low resistive component presented to the Unun by the radiating element), unwanted resonances when feeding highly reactive impedances and loss in the transformer core due to too low a value of shunt impedance when presented with a very high secondary impedance.

Perhaps surprisingly the Ununs with the lowest losses are not always the best type to use in this application.

This may seem odd, but high Q designs using Type 2 powdered iron cores tend to interact badly with some of the impedances that can be encountered at the base of the vertical radiator at specific frequencies. This interaction causes very high excursions of impedance which present an even worse match to the 50 ohm coax feed than would otherwise be present. Lower Q materials can help damp these excursions to manageable proportions. However it’s not a good idea to over do this, a balance has to be achieved. In some cases a 4:1 Unun (or Balun) doesn’t actually improve the match, or widen the matching range of a tuner. It’s just the additional loss masking the poor match. A 6dB attenuator in the feed line will allow you to achieve better than 2:1 VSWR on any band, but the match at the other end will still be bad.

See this graph showing the VSWR of several different 4:1 Ununs connected to a 6.5m vertical.

When required, I use a 4:1 Unun which I have specially optimised for this application, it is lossy but the improvement in match to 50 ohms offsets the mismatch loss which would otherwise occur along the feedline.

Using a 4:1 balun and remote coax fed tuner typically adds about 1.5 to 2dB of loss in the best case, compared to a coax fed ATU at the base of the antenna with no Unun. In the worst case with a bad set of parameters you can loose 10dB or more.

The bottom line is that you have to treat the antenna system as a whole in order to minimise losses.

Incidentally if you want or 'see' copper losses in an atu feeding a low impedance load, take a look at the thermal images towards the bottom of this page.

My experience is that most atu losses occur when trying to match to low impedance capacitive loads, usually when using short verticals on the LF bands.

Big external coils can help to improve this situation so that the atu doesn't have to work so hard.

If you take a look at this graph

Principle of operation is that by varying the pitch of the windings it should be possible to obtain multiple resonances or improved efficiency on lower frequency bands of operation relative to unloaded wire.

It is easy to obtain two resonances by adding a more tightly wound helical section on top of a less tightly wound section.

Ansoft software model

It may be possible to obtain multiple resonances by creating ‘trapped helical’ either by interwinding capacitance or by adding capacitive loading or patches to closely wound section of helix, however the operating bandwidth is likely to be very narrow.

Commercial products include these based on the DG7PE design.

MyDeL version of DG7PE

Chameleon V1

I have major doubts about the efficiency of these designs, as the construction technique is likely to introduce large resistive losses in the windings, and dielectric loss in the GRP support pole.

The suggested use of a 9:1 Unun is also of concern, as I believe that this will also introduce national losses which will mask the true impedance presented by the antenna to the transceiver. I estimate that the total antenna gain is likely to be in the region of 10 to 16db relative to that of a ¼ wave vertical.

However it may be possible to apply variable pitch helical loading to a larger vertical antenna, especially when used in conjunction with an auto-atu. The choking effect of the wound sections could reduce the active length of the antenna at the higher frequencies, whilst improving the efficiency at lower frequencies.

I would be very interested to hear from anyone who has used one of these antennas especially if they are able to measure the impedance with a VNA or antenna analyser. Detailed photos of the windings would also be appreciated, as I would like to try and construct an EZNEC model of the antenna.

M. Ehrenfried – G8JNJ – V1.0 – 1/06/2009

Information on this project can now be found here

Information on this project can now be found here