Having recently built a MTR2 3-band CW transceiver, I was keen to make band switching on a mountain top as simple as possible.
I have been using a 40m End-Fed Half Wave (EFHW) antenna very successfully for the last few months and I wanted to extend the design to multi-band whilst avoiding the need to take a tuner along. The matching unit I use requires no tuning which makes setup very quick.
The simplest method of making an EFHW multi-band is to put links in it to allow the antenna to be adjusted in length easily. This is OK, but is likely to require lowering a squid pole to reach the links to change bands – not ideal, but very simple.
The other method is to segment the radiator wire with traps for the various desired bands (noting here that I’m considering non-harmonically related bands). The challenge for a lightweight antenna is how to build traps that are compact, light and reasonably robust.
In the end I decided to build the latter and to build it to cover five bands: 40/30/20/17/15m. This was due to the fact I have my eye on a compact 5 band SSB/CW rig that covers those bands (the LNR Precision LD-5).
To keep trap losses low, it’s important to keep the component Qs as high as practical and also allow for the voltages and currents that appear across the traps at planned operating power levels. I used a target power level of 100W so that I could use it with a typical 100W transceiver if necessary.
I decided that to keep the traps as small as possible, I’d look for surface mount Hi Q transmit-rated capacitors and then build the coils around them. I found some excellent capacitors in the form of the ATC 100B series. These are porcelain dielectric SMD devices rated at 1kV and are good for more than 10A. However, they are not cheap at around A$2.40 each! Getting them is also challenging and the only place I could find them available in small quantities was from RF-Microwave.com in Italy (here’s the link: RF-Microwave).
I found an excellent coil former in the guise of a 4mm “rigid riser tube” used for micro-irrigation systems and widely available at Bunnings/Masters etc. These are about 4mm ID and have an OD of 6.4mm – the ATC capacitors fit comfortably inside this tube.
I wound the coils with 1mm enamelled copper to keep the RF resistance low and fed the ends back into the tube to make the connections to the capacitor. Soldering the capacitors on is a little tricky and you need to be careful not to put too much strain on them as they are only rated for 5 lb maximum tension on the end caps (I know, I broke one).
The following photo illustrates a completed coil winding with capacitor inside together with one of the capacitors for comparison.
The traps were aligned using my N2PK Vector Network Analyser by adjusting the turn spacing to achieve resonance at the required frequencies. I found that adding protective heatshrink over the trap dropped the resonant frequency by up to 60 kHz, which required some adjustments to get them right. The frequency is also very sensitive to the coil spacing due to the low L, high C trap design. I kept the inductance low to keep the coils physically small.
Here’s the details for the traps:
|Band||Measured resonant freq [MHz]||C [pF]||L [nH]||Turns|
Note that the number of turns in the table above are for a 6.4mm diameter former. A larger former would need less turns.
As you can see from the photo above, these traps are not designed to be strung in-line without some additional strengthening. To keep with the philosophy of ultra-light and ultra-compact design, I went searching for clever methods of strain relief and found on the AT-Sprint Yahoo group a reference by WT5RZ to a neat method: using braided fishing line to take the strain. It is quite simple in theory: use a nail knot to attach the braid to the antenna wire on either side of the trap leaving a little slack in the wires attaching to the trap.
- Use thick braid for both strength and also to prevent it cutting in to the wire insulation too much – I used 50lb braid
- Put some heatshrink over the wire to add both something soft for the braid to grip and also to protect the antenna wire insulation from the braid
- Be careful with a heat gun around braid – it melts. Better to apply the braid after all the heat shrinking is done to avoid weakening the braid
The wire lengths were calculated using a 4nec2 model and came out as follows together with the final adjusted lengths after trimming (note that they are significantly longer than modelled):
Note that the modeled wire is a DX-Wire like wire of Chinese origin that has a PE sheath and 6-stranded plated copper wires with an aramid strengthening element. Outer diameter is approx 1.6 mm and it is very light at 4 g/m. If you use a different type of wire, then these lengths could be significantly different.
The NEC2 model is at the bottom of this page if anyone wants to play with it.
The plot above was created with Z-Plots from Dan Maguire AC6LA.
As you can see, the SWR is better than 2:1 for parts of each of the bands of interest using a fixed EFHW matching unit (described here). It will be interesting to check it out in the field as I’ve found that SWR on an EFHW is dependent on both the surrounding environment and the height of the antenna ends from the ground. It is also dependent on the coupling of the feedline to the ground (given the matching unit I built uses the feedline as the counterpoise – I’ve found this works fine in practise in the field).
I should be able to use the upper portions of 20m and 15m with the help of an ATU if necessary. The detailed SWR plots for each band are in Appendix 1 below.
Here are some photos of the completed antenna rigged to a squid pole in the garden.
The end support cord for the antenna is an interesting product called Zing-It. This is sold for arborists to use with throw weights to get them into trees. As such it is incredibly abrasion resistant and strong. It’s 1.75mm diameter and has a rated load of 230 kg! It is also very hard to cut with a knife – even a Stanley knife had to be used like a saw to cut through it. It weighs only 1.65g/m and is available in Australia through Tier Gear in either yellow or grey.
I now need to try out the antenna in the field and see how well it works in practise!
Appendix 1 – Detailed SWR plots
Here are the detailed plots for each band for reference:
Appendix 2 – NEC2 model
Here’s the NEC2 source for the model. Note that it is modelled as a vertical with the feedpoint between the bottom of the antenna and ground. I haven’t found a better way to model an EFHW in NEC2. Thus it will not be a good model of the radiation pattern when deployed as an inverted V, but seems to be OK to calculate lengths of wire segments. Also note that you need to set the characteristic impedance of the system to whatever you think the end impedance of the EFHW may be – I found 4700 ohms was OK for modelling purposes – you are really looking for SWR minima to optimise the antenna segment lengths rather than predicting the absolute SWR value.
If anyone has a better way of modelling an EFHW, I’d be keen to hear it!
Here’s the model:
CM Trapped EFHW for 40/30/20/17/15m CM CE SY L15=6.622073 'Length of 15m element SY L17=0.819581 'Length of 17m element SY L20=2.144196 'Length of 20m element SY L30=2.459585 'Length of 30m element SY L40=5.313973 'Length of 40m element SY Ltrap=0.043 'Length of trap SY L15s=0.043 'Start of 15m element SY L15e=L15 'End of 15m element SY L17s=L15e+Ltrap 'Start location of 17m element SY L17e=L17s+L17 'End location of 17m element SY L20s=L17e+Ltrap 'Start of 20m element SY L20e=L20s+L20 'End of 20m element SY L30s=L20e+Ltrap 'Start of 30m element SY L30e=L30s+L30 'End of 30m element SY L40s=L30e+Ltrap 'Start of 40m element SY L40e=L40s+L40 'End of 40m element GW 1 100 0 0 L15s 0 0 L15e 0.00025 GW 2 1 0 0 L15e 0 0 L17s 0.00025 '15m Trap GW 3 10 0 0 L17s 0 0 L17e 0.00025 GW 4 1 0 0 L17e 0 0 L20s 0.00025 '17m Trap GW 5 20 0 0 L20s 0 0 L20e 0.00025 GW 6 1 0 0 L20e 0 0 L30s 0.00025 '20m Trap GW 7 50 0 0 L30s 0 0 L30e 0.00025 GW 8 1 0 0 L30e 0 0 L40s 0.00025 '30m Trap GW 9 50 0 0 L40s 0 0 L40e 0.00025 GW 10 1 0 0 0 0 0 L15s 0.00025 'Feedpoint GE 1 LD 7 0 0 0 2.4 0.00025 'DX-Wire UL wire LD 6 2 1 1 100 259nH 220pF '15m trap LD 6 4 1 1 100 351nH 220pF '17m trap LD 6 6 1 1 100 579nH 220pF '20m trap LD 6 8 1 1 100 528nH 470pF '30m trap GN 2 0 0 0 4 0.003 EK EX 0 10 1 0 1.0 0 0 FR 0 0 0 0 14.1 0 EN