In ‘Antennas’, November 2005, Irecommended the multiband doublet as themost flexible and practical solution for aportable antenna. The main advantage ofthis antenna is that the doublet length is notat all critical, a major advantage whenplanning an antenna for a location that youhave never seen. I also noted that a 20mcentre-fed length, supported in the centreusing a collapsible telescopic fibreglass pole,worked very well for all bands 40m to 10m.

I suggested that the antenna be fed withblack 300 slotted-line twin feeder, on thegrounds that it is very convenient forrunning through gaps in doors or windowsthat might be encountered in a location notseen before.

FEEDER LINE LOSSES. I received an e-mail from Brian Austin, G0GSF,commenting on this antenna and its feedarrangement. He says: “There’s a much moreimportant reason not to use coax and that’sthe very high SWR (easily > 10:1) likely tooccur at some frequencies (which ones willdepend on the antenna length). With coax,the increased line loss, over an above itsinherent loss/m, can be very significant andwill soon turn the transmitted signal into QRPor even QRPP, depending on what youstarted out with! Likewise, the receiver willsound very flat on those bands. By contrast,slotted line has a considerably lower intrinsicloss and a higher characteristic impedancethan coax. The end result is that the SWRexcursions tend to be smaller (because of thehigher Z0) and the increased loss due to theinevitable mismatch on some frequencieswill be a lot less too.

“My 23m inverted-V, which is fed with300 slotted line, is very poor on 80m. Thereceiver sounds as if there’s 10dB ofattenuation permanently in circuit; whichthere is because the feed-point resistance ofthe antenna is so low and the SWR so highthat even the ‘low-loss’ line becomesincreasingly lossy on that band and is madeworse the longer the feed-line.

“So low-loss line is mandatory if the

matching network sits at the transmitter end,which is where it usually is.”

I was aware of the greater losses causedby using coax cable rather than slotted-linetwin feeder and described a problem I hadwith a multiband doublet in [1]. This alsoreferred to losses caused by SWR graphs inG3SEK’s ‘In Practice’ column [2], whichgraphically illustrated this. I also described

how replacing the existing 25m of coaxcable with 450 ladder line made asignificant predictable improvement to thehigher HF band performance of my centre-fed multiband antenna.

However, I didn’t think that the SWRproblem would be significant when using aportable HF antenna with a short length offeeder (say 5m) and decided to use availablesoftware to make a few calculations. Thecentre feed impedance of the antenna wascalculated using EZNEC4. The feeder losseswere then calculated using TLW

(Transmission Line Calculator for Windows)available on a CD with The ARRL Antenna

Handbook [3].My portable antenna was examined first,

comparing 5m of RG-58 (taking the viewthat lightweight coax would be used on aportable antenna) and comparing it with450 slotted twin feeder. Actually 300slotted line was used with my portableantenna, but it is not included in the libraryof feeders in TLW and 400 was the nearestI could get. The results are shown in Table

1. The SWRs column represents thecalculated SWRs at the antenna and at theATU respectively, 11/9 for example. You cansee that the greater the transmission line lossthe greater the difference between these twoSWR figures. Using slotted line, even on ashort 5m length, gives an average increaseon all bands of over 3dB in antenna gaincompared to using coax.

If we now look at a more permanentinstallation of, say, a 22m inverted-Vantenna fed with 15m, which is similar to

70

PETER DODD

37 THE RIDINGS, EAST PRESTON,

W SUSSEX BN16 2TW.

E-MAIL G3LDO@UKONLINE.CO.UK

G3LDOANTENNAS

Coax totransceiver Coax

Normal ATUtwin feed

Conventional T network ATU

Antenna

Ladder line feed to antenna

External balun

Internal balun

FIGURE 1

© RADCOM 644

FIG 1: A CONVENTIONAL HIGH-PASS T-NETWORK TUNER WITH A BALUN FOR CONNECTING TO A TWIN FEEDER. WITH THIS ‘COMUDIPOLE’ ARRANGEMENT, THE INTERNAL BALUN CAN BE BYPASSED ANDA SEPARATE BALUN PLACED OUTSIDE THE SHACK AND CONNECTED VIA A SHORT LENGTH OF COAX.

THE MFJ-989C HIGH-PASS T-NETWORK TUNER WITH A FERRITE-CORED BALUN FOR A TWIN FEEDER.

Feeding doubletsThis month, G3LDO looks at losses on twin-line and coaxial feeders.

71

the G0GSF’s antenna, how do thetransmission line losses compare? Using thetransmission line shown in Table 2, we canexpect an average increase in gain of around5 or 6dB using slotted line rather than goodquality RG-213. G0GSF noted that on 80m“The receiver sounds as if there’s 10dB ofattenuation permanently in circuit”. FromTable 2 the calculated value of the slottedline loss is just over 2dB, which isn’t a highenough loss to be that noticeable on shortskip 80m contacts. There could be anotherreason for the poor performance.

While the efficiency of an electricallyquarter-wave short dipole is quite reasonable,being only about 1dB down on a full half-wave, the feed impedance is very low. In theexample shown in Table 2 the reflectedimpedance at the ATU, for our 22m antennawith 15m of slotted line feeder, is 4.0 +j40 .

Very low impedances can be matchedusing the conventional high-pass T-networktuner, but settings are rather critical and thelosses are very high. According to the ATUsection of TLW, the losses are in the regionof 5dB. The problem is made worse by the4:1 impedance step-up balun at the output.According to TLW, a low-pass L-network isrequired to match this very low impedanceto 50 . The problem can be avoidedaltogether by avoiding low impedanceantennas where possible – this is achievedin practice by using a doublet thatapproximates a half-wavelength on thelowest frequency in use.

LOSSES ON THE G5RV ANTENNA.

The G5RV antenna, which uses a section oftwin-line transmission line as tuned line,normally uses a length of coax from thebottom of the transmission line to the shack.So how efficient is this arrangement?

The calculations, shown in Table 3,indicate that losses can be quite high onmost bands, with the exception of 20 and12m. These results also indicate that anATU is essential if a G5RV is used as amultiband antenna. The figures in the SWRcolumn again show the SWRs at theantenna and the ATU respectively. Note thatthe thin RG-58 coax, with its higher losses,gives a lower SWR at the ATU whencompared with the better quality RG-213. Itfollows that if you have a nice low SWRmeasured in the shack on all bands theremight be something wrong with the coaxbetween the rig and the antenna.

A PRACTICAL METHOD OF USING

TWIN FEEDER. Most commercial ATUs usea high-pass T-network as shown in Fig 1.Most of these tuners can be used with twinfeeders by using an internal balun at theoutput of the tuner as shown in Fig 1,although this method of feeding twin lines isnot regarded with favour by some authorities[4]. However, as mentioned in [5], I haveused the small MFJ-901B with a 20m longdoublet and a twin feed. The balun in thisATU is mere 25mm OD toroid and itstemperature rose to just slightly aboveambient temperature after several hours of100W CW contest-type operation on variousbands, so it is obviously not that inefficient.

The main problem with twin feeder can berouting it into the shack. The feeder must notbe allowed to come in close contact withmetal objects or bundled together with otherfeeders or conductors. A solution wasdescribed by PA2ABV [6] in which the balun,normally located inside the ATU, is placed insuch a position that there is an RF obstacle-

free path between it and the antenna. The ATUis connected to the balun with a short lengthof coax cable as shown in Fig 1. Such anarrangement was called the ‘Comudipole’.Theinternal balun can be removed from the ATUand used externally or a separate balun can beconstructed, which is very simple, see [7]. Abalun wound on a T200 ferrite toroid can bewaterproofed by simply smearing it in grease.

PA0SE describes a balun constructed withcoax cable on a coil former [7], which isclaimed to have a better performance overferrite-cored baluns when handling the highlyreactive impedances seen in Tables 1 and 2.

TABLE 1

Calculated losses on 5m of transmission line feeding a 22m-long inverted-V,

comparing RG-58 with 450 slotted line.

Band Centre Z RG-58 450 slotted line

MHz R ± j SWRs Loss dB SWRs Loss dB

7.0 101 +205 11/9 0.18 5.1/5 0.02410.1 700 +365 69/29 2.9 8.8/8.7 0.02714.2 828 -1820 94/30 5.5 12.3/12 0.12318.1 107 -264 15/11 1.8 5.46/5.41 0.06621.2 200 +488 16/29 2.8 5.28/5.22 0.05125.0 1329 +688 79/23 5.4 9.77/9.52 0.12128.5 298 -880 56/20 4.35 8.37/8.18 0.099

REFERENCES

[1] Antennas, RadCom September 2002.

[2] ‘Losses from High SWR’, G3SEK, ‘In Practice’ RadCom,

March 2002.

[3] ‘TLW Transmission Line Program for Windows’, by N6BV, The

ARRL Antenna Book, 19th edition.

[4] HF Antennas for all Locations, L A Moxon, G6XN, 1984

edition, p54.

[5] ‘Antennas’, RadCom November 2005.

[6] Electron, December 1992, reported in ‘Technical Topics’

RadCom, July 1984.

[7] Backyard Antennas, G3LDO, RSGB.

TABLE 2

Calculated losses on 15m of transmission line feeding a 22m-long inverted-V,

comparing RG-213 with 450 slotted line. *The impedance at the ATU was 4.0

+j40 as calculated using TLW.

Band Centre Z RG-58 450 slotted line

MHz R ± j SWRs Loss dB SWRs Loss dB

3.6 12 -810* 600/44 14 148/106 2.17.0 101 +205 11/8 1.2 5.1/5.0 0.065

10.1 700 +365 68/19 5.5 8.8/8.4 0.16214.2 828 -1820 95/18 7.0 12/11.5 0.32418.1 107 -264 15/8 2.7 4.46/5.28 0.15521.2 200 +488 28/11 4.0 5.28/5.1 0.1525.0 3829 +688 79/13 7.8 9.77/7.88 0.3428.5 298 -880 72/12 6.7 8.3/7.8 0.30

TABLE 3

Calculated losses on 15m of coax feed to a G5RV antenna, comparing RG-213

and RG-58. The impedances have been calculated at the point were the coax

feeder is connected to the twin tuned line.

Band R ± j, tuned RG 213 RG58

MHz line end SWRs Loss dB SWRs Loss dB

3.6 25 +260 64/27 3.1 64/21 3.47.0 202 -444 24/14 2.3 24/10.8 3.7

10.1 337 +966 65/19 5.2 65/13 6.214.2 104 -4 2.1/1.9 0.6 2.1/1.8 0.818.1 322 -706 37/12.8 4.8 37/9.5 5.821.2 235 -684 44/12.8 5.3 44/9.3 6.925.0 177 -131 5.5/4.2 1.4 5.5/3.7 1.928.5 1233 -1215 49/11.6 6.1 49/8.5 7.6